Authorities recovered a package that apparently was mailed by James Eagan Holmes after the shooting suspect told investigators to look for the item on the University of Colorado Anschutz Medical Campus, a senior law enforcement official told NBC News on Wednesday.

The source told NBC that the package contained writings about killing people, but could not go into more detail.

Holmes, a 24-year-old who was in the process of withdrawing from a graduate program in neuroscience at the university, has been arrested in connection with the killing of 12 and the injuring of 58 in a shooting spree at the midnight premier of the Batman movie "The Dark Knight Rises" in Aurora, Colo. on Friday.

Police recovered the package on Monday after getting a search warrant for the medical center mail room and then getting a second warrant to actually open the package, the law enforcement source said. (Prosecutors later said in court documents that there was only a single search warrant.)

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Police found the package as Holmes described it, including his name in the return address, and it's now being analyzed, the source said.

It was unclear how long the package had been in the mail room before its discovery. 

In response to reporters' queries about the package, the Anschutz Medical Campus issued a statement saying it could not comment on anything regarding the ongoing criminal investigation into the theater shooting, under order of Arapahoe County District Judge William Sylvester.

The statement did provide general information about how mail is handled on campus.

"The University centrally receives mail from the United States Postal Service. The University then delivers the mail to the address on the Anschutz Medical Campus the same day it is received," the statement said. "The University's mail service is not open on Saturday. Saturday mail is sorted and delivered Monday morning. The University does not log or track mail/packages unless it requires a signature from the United States Postal Service."

Holmes made his first court appearance before the court in Arapahoe on Monday, amid grieving for the victims.

Wearing a red prison jumpsuit, Holmes appeared with public defender Tamara Brady for the hearing. Holmes, who said nothing during the proceedings, had several days' beard growth and bright red dyed hair. He looked down or off into the distance, at times raising his eyebrows in a quizzical expression or frowning as if concentrating.

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A hearing of formal charges, expected to be multiple counts of first-degree murder, is set for next Monday. Then the state must decide whether to seek the death penalty.

Holmes had been stockpiling ammunition, weapons, body armor and explosives for months, said authorities who tracked his purchases. After his arrest, teams of experts worked for two days to disarm Holmes' apartment, which contained an elaborate web of explosive and incendiary devices set to be triggered by tripwires.

NBC News' Kate Snow and Kari Huus contributed to this report. 

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ANALYSIS

While lessons learned from previous mass shootings may have helped limit the carnage in the Aurora, Colo., theater shooting, it remains unclear whether there were warning signs that might have been missed.  

Although it is too soon after the tragedy to draw any hard conclusions (unlike the shooting that took but a few minutes, investigations of such events are meticulous, time consuming affairs), it seems likely that lessons learned from similar events might well have reduced the human toll, which currently stands at 12 dead and 58 injured. 

It appears that first responders in Aurora arrived on the scene no more than a minute or two after calls began flooding 911.  That rapid response meant that the suspect was quickly detained, possibly preventing additional casualties. In addition, emergency medical care arrived shortly after, undoubtedly comforting and likely saving many of the wounded.  And in the coming days, counselors and other medical professionals will surely provide support to both the countless physical and emotional victims of the early morning shooting.

Each of these efficient and effective responses is born of past tragedies.  Through similar events close by (Columbine) and far away (the 2008 terrorist attack in Mumbai, India), first responders learned of the need for rapid intervention, securing a perimeter and waiting for specialized units.  Whether it is an active shooter scenario or a Mumbai-style terrorist attack, state, local and federal authorities have modified their tactics and training to fit what has regrettably become a semi-regular occurrence. 

Hard questions remain
But early indications are that some other lessons of the past may be more elusive.  Much focus has of course been on the shooter, and indeed it is critical that his past behavior be closely examined for warning signs that might have been missed.  In some cases, terrorist or non-politically motivated shooters display outward signs of future violence —either through antisocial behavior or obtaining weapons or their precursors — potentially putting officials in a position to disrupt the attack before it happens. As last week’s report on the Fort Hood shooting noted, however, even when such warning signs are present, our ability to identify them properly and take action is mixed. 

Such is surely the case as well for non-terrorism-related violence, as criminologists have long struggled with accurately identifying future criminals. 

That can be a slippery slope. If concerns about “false positives” are discounted, then we are more likely to intervene in the lives of those who show warning signs, but are merely eccentric, not dangerous.

In short, although we might be able to improve our ability to spot the next mass shooter or serial killer before the crime is committed, attempting to do so raises numerous legal and ethical questions and likely won’t be easy or particularly effective.  It hasn’t worked in the past and it is unlikely to work in the near future.

One need not be a terrorism expert or criminologist to know that every society has faced criminals and mad men before.  Are there more terrorists or mass murderers today than in the past?  Perhaps.  But what is unquestionably true is that it has become increasingly easy for murderers of any stripe to kill more and more people in one fell swoop.

In some parts of the world, terrorists have had to be innovative to do so.  The 2005 al-Qaida-inspired terrorist attacks in London, for example, used improvised explosive devices composed in large part of concentrated hydrogen peroxide.  Creating these weapons wasn’t excessively difficult, but doing so required some care. More than a few trained terrorists have tried and failed in the past, both in the U.S. (Faizal Shahzad in Times Square in 2009) and the U.K. (the bombers of July 21, 2005).  Early reports also allege that Holmes engaged in such explosives plotting. 

In the U.S., however, counterterrorism officials and police have long since accepted that neither creativity nor immense skill is required for an individual to be an effective mass murderer. The ready availability (either legally or illegally) of semi-automatic weapons with high-capacity magazines makes such killing remarkably easy.  Moreover, given the endless supply of “soft targets” in our open society, it is simply impossible to harden every theater, school and workplace. 

When I served as the director of the National Counterterrorism Center, whenever I was asked, “What keeps you up at night?” I without fail focused on a lone wolf who would be difficult if not impossible to detect and armed to the teeth with deadly firearms.  And while some of my foreign police and security counterparts had similar fears, all were sympathetic to the vastly greater challenges we faced in the U.S., where such weapons are so easily accessible. 

Looking ahead
None of this provides an easy answer.  Would stricter control of assault weapons and high-capacity ammunition magazines in the U.S. reduce mass murders?  Perhaps, but the current inventory is so vast that we might only be closing the proverbial barn door. Would an armed moviegoer have been able to stop the Aurora shooter in his tracks?  Again, perhaps, although given his preparations and the mayhem it would surely have been difficult for even a highly trained individual to do much good.  Needless to say, addressing any of these issues is made even more difficult by the political swirl around gun control -- not to mention the very real limitations of our Constitution’s Second Amendment. 

That being said, we must be honest about the challenges we face. We will never identify and stop all of the crazed killers before they strike, whether they are motivated by al-Qaida or something even more mysterious within their heads.  We can detect some through community-focused efforts, but there is no panacea or even anything close.  In addition, while we can reduce the carnage, no system of first responders will make us perfectly safe. 

While the truly committed killers will always find ways to kill, we regrettably live in a society where even the near-spontaneous, untrained individual can bring tragedy to the doorstep of entire communities.  Our freedom -- to live in a non-police state, to have a degree of privacy and most especially to bear arms -- has very real costs.  The destructiveness of certain firearms in the hands of some individuals increases those costs. We must all pay the price -- but none moreso than the victims in Aurora and elsewhere. 

Michael Leiter is a former director of the U.S. National Counterterrorism Center and NBC News consultant.

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• Photos of James Holmes, camp counselor for underprivileged kids

NBC News

James Eagan Holmes, right, goofing around with an unidentified fellow counselor at Camp Max Straus in summer 2008, near Glendale, Calif.

James Eagan Holmes, the suspect in the mass killing in Aurora, Colo., was a counselor in the summer of 2008 at a residential camp for underprivileged children near Glendale, Calif.

Jewish Big Brothers Big Sisters of Los Angeles confirmed that Holmes was a cabin counselor, responsible for 10 children at its Camp Max Straus for children ages 7 to 14 from Los Angeles. Holmes was then a 20-year-old student at the University of California, Riverside, and neighbors have said he was active in the Presbyterian church that the family attended. The camp is nonsectarian.

A statement from the group said, "His role was to insure that these children had a wonderful camp experience by helping them learn confidence, self esteem and how to work in small teams to effect positive outcomes. These skills are learned through activities such as archery, horseback riding, swimming, art, sports and high ropes course."

A fellow counselor told NBC News that Holmes seemed shy.

"The entire staff was really close, considering we lived together, except for James," said the counselor, who asked that she not be named. "He really kept to himself and hardly ever went on any trips with the rest of the staff. He was very shy and reserved."

Photos of the staff show Holmes goofing around with other counselors.

"It is sickening," the fellow counselor said, "knowing that he killed kids the same age that he once cared for." The youngest of those who died in Friday morning's shooting is Veronica Moser-Sullivan, 6. Holmes, who has not been charged with a crime, is scheduled to have his first court appearance on Monday and is expected to face 12 counts of homicide and many counts of attempted homicide.

The CEO of Jewish Big Brothers Big Sisters of Los Angeles, Randy Schwab, told NBC4 Los Angeles that Holmes had no disciplinary problems. "It is with shock and sorrow that we learned of the incident in Aurora," Schwab said. "Our hearts and prayers go out to all the families and friends of those involved in this horrible tragedy."

For more on what's known about James Holmes, read our earlier story, Suspect was buying guns, dropping out of neuroscience program.

More reading: Last year, after the shootings of Rep. Gabrielle Giffords and others in Tucson, we explored the question, is there a "type of person" who carries out such an attack? A study by the U.S. Secret Service sheds some light, and you may be surprised at the answers. Read that earlier story here: Few assassins fit the 'profile.' Most had no mental health treatment, made no threats.

Have information?
Do you know James Holmes? If you have information, send an email to Bill Dedman of NBC News.

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Updated Friday 9:25 p.m. ET: James Eagan Holmes, suspected of carrying out the Colorado movie theater shooting while wearing an outfit of black ballistic gear, was a Phi Beta Kappa graduate student in neuroscience who started buying his four weapons legally in May, about the time his grades fell and he began the process of dropping out of school.

A law enforcement official confirmed that Holmes had two handguns, a shotgun and a semi-automatic rifle, had his hair brightly colored red or orange, and told police that he was the Joker, the fictional villain in earlier Batman comics and films. Holmes is not cooperating with authorities, other than to divulge that his apartment was rigged with explosives. He is represented by an attorney.

One difficulty for investigators is that the explosives in the apartment of the only suspect in this shooting are making it difficult to get to his computer, any writings or other information that could explain motive, why he apparently committed this mass killing. Police called it a "vexing problem." Police would say nothing about a possible motive or what Holmes' demeanor has been. Police have suspended for the night their attempts to enter his apartment.

In the past 60 days, police said, Holmes bought more than 6,000 rounds of ammunition, at gun shops and over the Internet.

The 24-year-old from San Diego, known to friends as Jimmy, was a Ph.D. student at the University of Colorado Medical School campus in Aurora, a university spokesman told NBC News.

"The University of Colorado Denver Anschutz Medical Campus can confirm that Mr. James Holmes was in the process of withdrawing from the University of Colorado Denver's graduate program in neurosciences," the university statement said. "Mr. Holmes enrolled at the university in June 2011."

A poor last semester
The Washington Post reported that a neuroscience faculty member at Colorado who said he taught Holmes said he immediately thought of Holmes when he heard that a student was accused of the shooting. The faculty member said Holmes was "very quiet, strangely quiet in class" and seemed "socially off."

Holmes did very poorly on his comprehensive exams last semester, the instructor told the Post, and the school was considering placing him on academic probation, but was not considering expulsion.

The university website listed one of his courses as the Biological Basis of Psychiatric and Neurological Disorders. He was listed on the class website as making a presentation in the spring on MicroRNA biomarkers.

The University of California, Riverside, confirmed that a student named James Eagan Holmes, with the same date of birth, graduated with a bachelor's degree in neuroscience in 2010. He graduated in four years, attending from the fall of 2006 to spring 2010. Public records show that the Holmes living in Aurora had a previous address at a Riverside dormitory.

Have information?
Do you know James Holmes? If you have information, send an email to Bill Dedman of NBC News.

'A little strange,' 'very smart'
A student who lived across the hall from Holmes at Cal-Riverside, who asked not to be named, said Holmes completed the honors program and was a member of Phi Beta Kappa and Golden Key honor societies.

"I always thought that he was a little strange. I could never put my finger on it, but something told me to not get too close to him, female instincts I guess," the female student told NBC News. "I had tons of classes with him and lived across from him in the Honors dorms. He was a very smart guy though. He was a little bit of a weird guy, but we were honors students, so weird people were kind of common."

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Aurora Police Chief Dan Oates said Holmes, born Dec. 13, 1987, is the man who is believed to have killed at least 12 people early Friday at a midnight showing of the new Batman movie in Aurora, a suburb of Denver. At least 58 other people were injured, nearly all of them by bullets but a few in other ways in the chaos, Oates said.

Holmes has not yet been charged with any crime and will appear in court on Monday.

"We are confident that he acted alone," Oates said. He said he had no way to know yet how many rounds were fired, but it was "many, many." CNN said the gunman had a magazine that would have contained more than 100 rounds.

Holmes was arrested without any resistance at his white Hyundai car in the theater parking lot, parked just outside the theater's back door.

University of Colorado

James Holmes, the suspect in the Aurora, Colo., movie theater shooting.

He was wearing a black ballistic or bullet-resistant helmet, a ballistic tactical vest with pockets, ballistic leggings, throat and groin protectors, a gas mask and ballistic tactical gloves, Oates said.

Weapon purchases started in May
In addition to two canisters, perhaps holding teargas, four weapons were found at the scene, Oates said.

• Two were .40-caliber handguns, made by Glock. At least one of those was used, the police chief said. Holmes had purchased more than 3,000 rounds of .40-caliber ammunition, Oates said.
• One shotgun, a Remington Model 870, one of the most popular models. Pump action, single barrel, 12 gauge, with 300 rounds.
• And one Smith & Wesson AR-15 type rifle, .223 caliber, called by some an "assault rifle." These weapons can accommodate large ammunition clips, and Holmes had one "drum clip" that would have carried more than 100 rounds, Oates said. With that clip, he could have fired 50 to 60 rounds in a minute, even if the rifle was considered semi-automatic, not automatic, Oates said. He had 3,000 rounds of ammunition for this rifle.

Officials told NBC News that all four were purchased legally, beginning in May, from two national chain stores: Gander Mountain Guns and Bass Pro Shops.

Bass Pro Shops released a statement saying that employees at a Denver store followed all laws when they sold two weapons to Holmes. "We want to offer our deepest sympathies to the victims and their families," said Larry Whiteley, manager of communications for the company. "This is an unspeakable tragedy, and we join with all Americans in offering our prayerful support. Based on the records we have reviewed, personnel in our Denver store correctly and fully followed all Federal requirements with respect to the sale of one shotgun and one handgun to the individual identified in this incident. Background checks, as required by Federal law, were properly conducted, and he was approved. Again, our hearts go out to the victims and their families. We also offer our support and appreciation to the law enforcement and emergency response professionals and all others who responded to give aid to these innocent victims."

CNN spoke with the CEO of TacticalGear.com, which said it sold Holmes a Blackhawk urban assault vest for $107, along with a triple pistol magazine, an M16 magazine pouch and a silver knife.

The only previous police record for Holmes is a speeding ticket in October 2011, the chief said.

Family statement
Holmes' family, who live in Rancho Penasquitos, a well-to-do suburban community in the northeastern part of San Diego, issued a statement through the San Diego Police Department.

"Our hearts go out to those who were involved in this tragedy and to the family and friends of those involved," the statement said. "We ask that the media respect our privacy during this difficult time. Our family is cooperating with authorities in San Diego, California, and Aurora, Colorado. We are still trying to process this information and we appreciate that people will respect our privacy." A man believed to be Holmes' father, James, was seen leaving with luggage, escorted by police. The Associated Press said the father is a manager at a software company, and his mother a nurse.

A neighbor, Tom Mai, told reporters on the block that Holmes was a shy, well-mannered kid, clean cut and responsible, who was very active in the church. The Associated Press reported that the family attended a Presbyterian church and threw a quiet Christmas party for neighbors. Holmes had trouble finding work after college, Mai said, and then went off to graduate school.

Rooting for the villain?
Holmes attended Westview High School in San Diego, graduating in 2006, the Poway Unified School District confirmed. Classmates showed yearbooks with his photo on cross country and soccer teams.

NBC 7 San Diego spoke with a classmate, Sumit Shah, who said he went to school with Holmes. "He was pretty shy, but once he got comfortable with you, he was the funniest, smartest guy… He always had something witty to say." He continued, "The guy I knew in high school, I don't understand how that could be the same guy…He was shy and little quiet, but he was never aggressive or mean. He always had really good grades. He seemed pretty normal."

A woman who said she knew him in high school told NBC News that Holmes was a good person, but oddly always rooted for the villains in superhero movies.

"He was a nice guy. Who very much wanted to be liked and wanted," the woman said. "He was a very, very smart guy. I honestly can not believe he could do this. I know, I know, everyone says that. But it is truly devastating to me.

"He did not have many friends for someone who wanted to be liked," she said. "He loved all the villains in superhero stuff, which I did point out as odd. Most people enjoy the hero!"

Her cousin, who knew Holmes and played soccer with him, offered this assessment by email: "Jimmy was kind to those who knew him. It was hard to get to know him, but once you did, you realized he was funny and accepting of everyone's faults. He loved video games. But I would say he did not like the shooting games. He preferred others, guitar hero. He was always quiet on the soccer field, but was committed to the team. Which is someone you want on your side even if they aren't the best. The last time I talked to Jimmy, he didn't really seem to be in a good place. But that was years ago. But nothing out of the ordinary, just you know the 20's trials and tribulations. What he did was horrible, but I will always know him as Jimmy Holmes, not this person he is being portrayed as. Just so devastating, maybe if I tried to keep in contact or something. Just hope and pray that the families that were involved know that everyone is thinking of them. I just am thinking of Jimmy also."

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• Families, friends anxiously await word of missing moviegoers
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• Tragedy in Colorado: 'We've had our share

'Quiet and easy-going'
Public records indicate that Holmes lived in the Aurora building where police have found explosives, at 1690 Paris St., Apt. 10. The building is reserved for students, faculty and staff from the medical campus.

The Denver Post reported that Holmes, in an apartment rental application he last year, described himself as "quiet and easy-going." A pharmacy student who lives in the building told The Post he called 911 around 12:30 a.m. Friday (2:30 a.m. ET) because there was a song blaring from the stereo inside apartment 10, where Holmes lived. The student, who wanted to be identified only as Ben, said he couldn't make out the song but that it seemed to be the same one playing on repeat. He also said Holmes kept to himself and wouldn't acknowledge people when they passed in the hall and said hello. "No one knew him. No one," he told The Post.

Melvin Evans, who was a bouncer at a karaoke bar near Holmes' apartment, said he recalled Holmes as a patron from checking IDs. He said Holmes would stroll into the Zephyr Lounge, sit quietly in a corner booth and have a Budweiser, but never joined in the singing. "He would just sit by himself. He wouldn't talk to anybody," Evans said. "He was really, really mellow, really calm. You wouldn't even look twice at him, if you passed him on the street."

Officials said Holmes was not on any watch list that would have alerted authorities that he was dangerous, officials said. The incident was not believed to have any connection to international terrorism, they added.

An earlier report that the car Holmes was driving had Tennessee plates turned out to be incorrect, officials said.

Watch live video from KUSA

'He looked so calm'
Police said the gunman entered through an exit door and appeared at the front of the theater in Aurora and released a canister, thought to be tear gas, that let out a hissing sound. He apparently had bought a ticket, propped open a door, and gone to his car for his gear.

The gunman then started shooting into the crowd, sparking pandemonium.

"He looked so calm when he did it," an eyewitness told NBC affiliate KUSA. "It was like scary. He waited for both the bombs to explode before he did anything. Then, after both of them exploded, he began to shoot."

"He had no specific target. He just started letting loose," the witness added.

Witnesses told reporters that the gunfire erupted during a shootout scene in the "The Dark Knight Rises." 

More on this story from breakingnews.com

After being captured by police, Holmes told authorities he had explosive materials in his apartment, KUSA reported.  

Local and federal officials searched Holmes' apartment building, which was evacuated soon after the shooting. 

Oates, the police chief, said the apartment had been booby-trapped with sophisticated explosives or flammable material. Officers were trying to determine how to defuse the devices.

Contributors to this report include staff from NBC News: Ian Johnston, Brinley Bruton, Sevil Omer, Suzanne Choney, Rosa Golijan, Joe Myxter, Petra Cahill, Kari Huus, Dick Belsky, Lauren McCullough, Michael  Brunker, Jason White, Bob Sullivan, Maggie Fox, Roland Jones, Carissa Ray, Alex Johnson, Gael Fashingbauer-Cooper, Will Femia, John Schoen, Allison Linn, Reena Joy Flores, Becky Bratu, Martin Wolk. The Associated Press and Reuters also contributed.

More related content from NBCNews.com:

• Raw audio documents eerie first moments of Colo. theater shooting
• Mass murderers often not mentally ill, but seeking revenge, experts say
• Woman who died in rampage narrowly escaped being shot last month
• Security at movie theaters comes into focus in wake of shootings
• People with same name as suspect hounded on social media
• Witnesses react online to 'Dark Knight' theater shooting

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