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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