Any major bomb investigation is an elaborate, high-tech piece of choreography by city, state and federal agencies with two goals: helping investigators figure out who built and planted the explosive device or devices and preserving evidence so that they can be brought to justice.

And the forensic investigation established soon after the twin blasts at Monday’s Boston Marathon has already begun yielding results. On Tuesday afternoon, law enforcement officials had indications that the bombs were made from pressure cookers filled with explosives, nails and BBs, and that they were placed in black nylon bags.

The work is far from over. Fragments, blood smears and explosive residue from the Boylston Street crime scene -- all of it will be carefully cataloged and examined. Every frame of video and every photo will be scrutinized, a mammoth undertaking in light of authorities’ pleas for spectators to turn over their images.

There’s other evidence to consider, too. A smell of sulfur in the air could indicate smokeless black powder was used. The size and color of a fireball could point to certain additives. Certain bomb mechanisms -- a type of fuse, a type of timer -- could be signatures of a particular group.

Alex Trautwig/Getty Images

A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during Boston Marathon on April 15.

"In an investigation of this nature, no detail is too small," Attorney General Eric Holder said Tuesday.

As the data piles up, investigators will begin to get some idea of who was behind the horrific act: an amateur or a professional bomb-maker, a home-grown lone wolf or a foreign-sponsored terrorist gang.

Early indications were pointing toward a sophisticated creator, as the bombs were designed and placed to act like a "homemade claymore," a powerful, directional anti-personnel device, sources involved in the investigation said.

These and other sources say that the triggering mechanism appears to have included a battery pack and a circuit board, the elements they said of a sophisticated triggering mechanism. Both of those elements were recovered at the scene.

"It appeared to be built from scratch but with a sophisticated triggering mechanism. And frankly, at the end of the day, all bombs are crude devices, and it is the way they are triggered that can be sophisticated," said one official with strong knowledge of explosives. "They functioned as designed."

In these kinds of investigations, the forensic process begins as soon as police have done what they can to preserve human life and clear the area, with bomb technicians and emergency service cops canvassing for devices that may not have exploded yet.

Protection from contamination
In Boston, officials confirmed they used controlled explosions -- usually done with water cannons -- on five suspicious packages that turned out not to be bombs.

Afterward, uniformed officers -- in Boston's case, the National Guard -- secure the perimeter of the blast site to protect the evidence from contamination until the specialists can bag, tag and transport it to a central location, where a prosecutor would ideally be supervising the chain of custody, local and federal officials say.

Boston's FBI Special Agent in Charge Richard DesLauriers said that recovery effort officially began Tuesday morning.

In these investigations, the entire area is photographed and investigators begin a grid search, working outward from the seat of the blast, swiping fragments for explosive residue and gathering anything that could be a clue. In Boston, debris has been found on roofs and embedded in buildings. What’s recovered will be sent to the FBI lab in Quantico, where it will eventually be logged on a grid, according to law-enforcement officials.

The painstaking work can have big payoffs.

If authorities can identify a type of explosive, they can try to trace where it might have been purchased. 

As House Homeland Security Chairman Michael McCaul, R-Texas, noted, if the probe reveals the bomb was set off by a cellphone call, investigators can track all the calls made at exactly that moment.

Private and city surveillance cameras can show the color and density of the smoke plume -- details that can point to bomb size and ingredients.

Damage can tell a tale
To the trained eye, damage to the area and to the victims also yield important information. In Boston, pockmarks on the buildings pointed to a bomb packed with BBs. The gruesome injuries -- legs torn from bodies -- would indicate the bomb was close to the ground, experts say. The absence of widespread ear and lung injuries is associated with a low-explosive device.

Blood from the scene is also collected. The lab can later compare the samples to the victims' types to determine if there's a swab that has no match and could belong to a suspect.

If investigators are lucky, within 12 hours they will have enough fragments to pinpoint certain aspects of the bomb -- as has happened in the Boston case. The FBI then begins building a facsimile of the device.

Boston bombing investigators were working to identify the type of timer -- whether it was a cellphone alarm, for instance -- and verify other components and substances used.

The manpower required for such tasks is massive. A probe like the one in Boston could easily involve more than 100 people in forensic collection and analysis.

In Boston, the FBI has taken the lead. The Bureau of Alcohol, Tobacco and Firearms announced it was sending 30 specialists to the city. There will be experts in dental work, DNA, blood-pattern analysis and digital video analysis working the case. Local and out-of-town agencies devoted bomb squad and forensic personnel to the investigation.

Typically, 36 hours after a bombing the forensic teams will have collected and mapped what they can and will consider releasing the scene. The hope is that by then they will also have someone in custody or some idea of who it is they're hunting.

Richard Esposito is the author of “Bomb Squad: A Year Inside the Nation’s Most Exclusive Police Unit”.

Slideshow: Boston Marathon explosions

Charles Krupa / AP

See images from the scene of the explosions.

Launch slideshow

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This story was originally published on Wed Apr 17, 2013 3:39 AM EDT

The White House, facing fierce criticism from the gun lobby, has delayed approval of a proposed rule that federal law enforcement officials say could help them stanch the flow of U.S. assault rifles and other high-powered weapons to Mexico’s drug cartels.

The proposed rule, announced by Alcohol, Tobacco and Firearms acting director Kenneth Melson on Dec. 20,  would require U.S. firearms dealers in four southwest border states to report multiple sales of long guns, such as semi-automatic assault rifles which are frequently purchased by so-called “straw buyers” for the cartels. Melson had said he expected the proposed “emergency rule” would receive approval in early January 2011.

But the announced deadline date for White House approval, Jan. 5, has come and gone, leaving ATF officials bewildered and keenly disappointed. Some officials had expressed hopes  that President Barack Obama might even address the issue during his State of the Union speech Tuesday night as a positive step the administration was taking to address the issue of gun violence.

Instead, Obama failed to discuss guns in his speech, and now some ATF officials are wondering whether the proposed emergency rule will take effect at all.  One official with knowledge of the issue said the delay may relate to questions raised by critics about ATF's legal authority to issue such a proposed rule on an emergency basis.

“This is hugely demoralizing and embarrassing for ATF,” said one former agency official who has followed the debate over the rule closely, speaking on condition of anonymity.

U.S.: Gun raids show cartels at work in Ariz.

Reid Cherlin, a White House spokesman said the proposed rule is still “under review” by the Office of Management and Budget and declined to offer any guidance on when it might take effect, if at all. An ATF spokesman declined comment.

U.S. law enforcement officials said the need for the rule was dramatically highlighted this week by federal indictments in Arizona alleging that networks of “straw buyers” – many of them working for the Sinoloa Cartel and other Mexican drug trafficking organizations -- had illegally bought hundreds of firearms from U.S. gun stores. Out of 700 firearms allegedly illegally purchased by one network between September 2009 and December 2010, more than 640 were bought at a single gun store, the Lone Wolf Trading Co., in Glendale, Ariz., according to one indictment.  Most of the weapons were AK-47s, purchased in bulk quantities of 20 to 40, often by the same buyer within days of a previous purchase. In each case, the buyers filled out federal firearms affirming they were buying the guns for themselves and underwent standard federal background checks.  In fact, according  to  federal authorities, they were buying the guns in order to smuggle them to Mexico, where many were later recovered from drug cartel operatives.

 ATF officials say the proposed rule would be an invaluable “intelligence” tool that would allow them to at least identify suspicious activity at gun stores along the southwest border. Currently, firearms dealers such as Lone Wolf Trading are required to report to ATF whenever somebody buys two or more handguns within a five day period. But they are not required to file such reports in the case of long guns, such as AK-47s, even though such assault rifles are now the “weapons of choice” for the Mexican cartels, officials say.  In this case, the Lone Wolf gun store “did nothing wrong” by selling the AK-47s in bulk quantities, although from a law enforcement perspective, the multiple purchases "kind of hit you on the smell test,” said one U.S. law enforcement official, also speaking on condition of anonymity. 

The proposed rule has drawn strong e criticism from the National Rifle Association and gun industry groups, which have publicly urged members and supporters to file public comments expressing opposition to the White House Office of Management and Budget. (OMB, an arm of the White House executive office, must sign off on all proposed federal rules.)

Larry Keane, senior vice president of the National Shooting Sports Foundation, which represents the gun industry, said his group believes the proposed rule “won't assist law enforcement” and “will make it harder for firearms dealers to cooperate with ATF.”

“The cartels that are using straw purchasers will simply modify their behavior,” he said. “Instead of sending one purchaser to buy five firearms, they’ll send straw purchasers  to five different stores — or they’ll simply recruit five straw purchasers to buy one gun at a time.”

In addition, Keane said, “If ATF can ask for  this in these four states, they can ask for that nationwide and there’s no piece of information they can’t ask for.” 

The uncertainty about the rule has fueled the disappointment of gun control groups that Obama has failed to take any action to press for tighter gun control measures, even after the recent shooting rampage in Tucson that seriously wounded Rep. Gabrielle Giffords of Arizona. Many gun control advocates, such as New York Mayor Michael Bloomberg, had called on Obama to seize the opportunity to forcefully address the issue Tuesday night, pointing out that the president during the 2008 campaign had backed tougher measures, including reinstatement of the federal assault weapons ban.

Obama’s failure to say anything on the subject drew criticism from Bloomberg and other gun control advocates Wednesday.

“It’s depressing, but not surprising,” said Kristen Rand, legislative director of the Violence Policy Center, a group that advocates for gun control.

Responding to the criticism, White House Press Secretary Robert Gibbs suggested Wednesday that Obama may address the gun issue in the future. 

"I wouldn't rule out that at some point the president talks about the issues surrounding gun violence," Press Secretary Gibbs said aboard Air Force One on the way to an event with Obama in Wisconsin, according to the Washington Post. “I don't have a timetable or, obviously, what he would say."

More reporting by Michael Isikoff