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Boston Marathon bombs meant to act as 'homemade claymore,' sources say

FBI via AP

An image from a bulletin issued to law enforcement and obtained by The Associated Press shows the remains of a pressure cooker that the FBI says was part of one of the bombs that exploded during the Boston Marathon.

BOSTON – The deadly explosions at the Boston Marathon were caused by pressure cooker bombs placed to act like a "homemade claymore," a powerful, directional anti-personnel device, sources involved in the investigation told NBC News on Tuesday.

These and other sources say that the bombs appear 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, they said.

"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."

A day into the investigation of the terrorist attack, authorities were beginning to piece together clues that they hope will eventually lead to the perpetrators.

Publicly, authorities said little about possible leads at morning and afternoon briefings, providing definitive information only on the number of casualties and the fact that only two bombs had been planted, rather than three or more as some early reports had suggested.

In their first discussion of the makeup of bombs at the afternoon briefing, they said investigators had recovered forensic evidence that led them to tentatively conclude that they consisted of an explosive and shrapnel -- BBs and pieces of nails -- packed inside metal containers. One container was definitely a pressure cooker, and the other is similar but there was not yet sufficient evidence to determine conclusively that it was also a pressure cooker. 

The devices were believed to have been placed inside black nylon backpacks or sacks, which were used to transport them to the scene, said the officials. A person assigned to the case later told NBC News that the devices appear to have been delivered to the scene in duffel bags.

An image from a law enforcement bulletin showed a twisted piece of metal that the FBI said was from a pressure cooker that held one of the devices. Another image showed a fragment of material from the bag or sack that had held one of the bombs, according to the bulletin.

This image from a bulletin issued to law enforcement and obtained by The Associated Press shows the remains of a black backpack that the FBI says contained one of the bombs that exploded during the Boston Marathon.

The evidence was being sent to the FBI lab in Quantico, Va., for analysis, after which a final determination on its composition would be made, said FBI Special Agent in Charge Richard DesLauriers. 

Separately, law enforcement officials told NBC News that the explosive is believed to be a "low explosive" – likely black powder or smokeless powder. That means the charge unleashes energy that travels at under 3,300 feet per second when detonated and, as a result, does not create a "blast wave" effect that causes often-fatal injuries from compression and massive blast damage, such as broken windows and building facade damage at a considerable distance. That does not mean a low-power device, one official explained, noting that such devices have the power to propel shrapnel a great distance, injuring and potentially killing victims many yards from the "seat of the blast," as the spot where the bomb is placed is known.

According to federal officials, there currently is no indication of any overseas involvement or direction in the attack, and although the case is being handled by the International Terrorism section of the FBI, that is simply a matter of resources, and if it turns out to be domestic the command of the incident will simply be switched.

Investigators say pressure cookers packed with shrapnel were used in the Boston attack. NBC News' Jay Gray reports.

Crime scene processing continues and hundreds of hours of video are simultaneously being viewed, a federal official said. At some point FBI behavioral profilers will join in the analysis, officials said, as they begin their effort to put together who the person or persons were who launched the attack.

While determining the makeup of the device is a significant first step, DesLauriers seemed to acknowledge that a long road lay ahead of investigators in an opening statement at the morning briefing.

"The American public wants answers,” he said. “The citizens of the city of Boston and the Commonwealth of Massachusetts want and deserve answers. This group of dedicated men and women standing before you today pledge to do everything possible to get those answers.”

Massachusetts Gov. Deval Patrick put to rest one piece of bad information, saying at the outset of the briefing that only two bombs were found at the scene, contrary to early reports that unexploded bombs had been discovered nearby and detonated. Officials also said a fire at the JFK Presidential Library and Museum apparently was unrelated.

And Boston Police Commissioner Ed Davis set the number of casualties in the attack at 176, including three fatalities and 17 critically injured victims still being treated. Officials also said that at this point, none of the victims is considered a suspect.

But many other questions remained unanswered, or at least unrevealed as of late Tuesday, including:

-- Were the bombs made and planted by one individual? What motivated the attack? Why has no one taken credit? All are questions officials are attempting to answer. Officials asked the public at the briefing to submit photos and videos of the area around the finish line, but gave no indication that they were looking for a specific individual. What is known is that they are using forensic techniques, witness accounts and video from numerous points of view to attempt to re-create what occurred. 

State and federal law enforcement are out in force at the crime scene, with forensic specialists and federal agents looking for any evidence that could lead to information on who is responsible for the bombings. NBC's Pete Wiliams and Michael Leiter discuss the latest.

-- When were the bombs placed? Police officials said the finish line area was swept by security for explosives on the morning of the race and again an hour before the first runners crossed. The elite women began crossing just before noon, and the bombing occurred about 2:50 p.m. ET, leaving nearly three to four hours since the last sweep. 

-- Is there any connection between the bombing and the overnight search of an apartment in Revere, Mass., five miles north of Boston? Federal officers left just after 2 a.m., and reporters saw them carrying two black trash bags, a duffel bag and a grocery paper sack of unidentified material out of the apartment. They also questioned two men who arrived during the search. 

-- Is the attack connected either to a man seen leaving the blast scene wearing dark clothing and a hood or to a rental truck seen attempting to enter then area near the finish line before the blasts? Both were the subject of BOLOs – acronym for “Be On the Look Out” alert – issued by police after the bombings.

Richard Esposito is senior executive producer for investigations for NBC News; Pete Williams is NBC News' chief justice correspondent; Bill Dedman is an investigative reporter for NBC News.

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Jessica Rinaldi / Reuters

Officials take crime scene photos on Tuesday, a day after two explosions hit the Boston Marathon.