The forensic investigation of the Boston Marathon bombing is developing a preliminary picture of how it was done, though not yet who might have done it.
Almost immediately, the two billowing clouds of white smoke visible in videos were a tipoff to bomb specialists about the type of explosives used. Within 24 hours, enough bomb fragments were retrieved by investigators to suggest that at least one of the two improvised explosive devices was a metal pressure-cooker pot packed with explosives -- a low-tech design that has been promoted by al-Qaeda to its radicalized American followers.
Investigators are casting a wide net, examining the possibility of a foreign- or domestic-inspired attack, said Timothy Murphy, an FBI agent for 23 years who served as deputy director of the bureau for a year and a half ending in 2011, when he left to work in the private sector.
“They’ll look at the timing -- the events that occurred in the past, who is most likely to commit something like this,” Murphy said. “They’ll come up with theories. They’ll look at international terrorism, including state sponsored. Who else would potentially be involved here?”
The white smoke indicated that the bomber used so-called smokeless or black-powder explosives rather than a military-style high-explosive such as C-4, which produces a distinctive black smoke, according to Fred Burton, former deputy chief of counterterrorism for the State Department’s Diplomatic Security Service who investigated the first World Trade Center bombing.
Those clues can be tricky, however. It’s also possible the white smoke could be concrete dust if the bomb exploded in front of a white building, Burton said.
“The real way to know what explosive was used is to analyze the post-blast debris for traces of explosive,” Michael Sigman, assistant director for physical evidence at the National Center for Forensic Science at the University of Central Florida in Orlando, said yesterday.
Investigators can identify the chemical composition of the explosives quickly, he said in a phone interview. The analysis may provide some indications about its source, though that may be limited, particularly if it’s a commercial material, he said.
“The people doing these investigations are extremely knowledgeable and they have extremely sensitive instrumentation in the field and in the laboratory,” said Sigman, who is also associate professor of chemistry at the university. “If they didn’t already know this morning what explosives were actually used, I’d be surprised. They probably knew yesterday.”
The bombs may have been in pressure cookers, FBI Special Agent Richard DesLauriers told a televised news conference in Boston yesterday. Investigators found scraps of nylon at the site of the attack and suspect the explosives may have been carried in a heavily laden backpack or bag, DesLauriers said.
The FBI and Homeland Security Department yesterday circulated a bulletin to other law enforcement agencies showing a picture of a fragment of a pressure cooker that investigators believe was part of one of the bombs used in the attack.
The damaged metal pressure cooker, mangled and dented by the blast, was accompanied by a separate picture of a damaged black backpack that investigators believe may have held one of the two explosive devices placed near the race’s finish line.
The improvised bombs contained shards of metal, nails and ball bearings to increase the carnage. Some victims had 40 or more fragments of pellet and nail-like shrapnel embedded in their bodies, said Doctor George Velmahos, chief of trauma surgery at Massachusetts General Hospital.
The fragments were uniform, indicating that they came from the two blasts and not from the surrounding environment, he said in a separate news briefing yesterday.
The use of a metal pressure cooker -- a variant on the more common pipe bomb -- increases the lethality of the blast because of the metal shards.
“You’re trying to confine an explosive,” David Chipman, who worked for the Justice Department’s Bureau of Alcohol, Tobacco, Firearms and Explosives for 25 years, said yesterday. “The tighter something is contained when it explodes, the more ferocity of the blast.”
The Department of Homeland Security issued a 2004 warning about the “potential terrorist use of pressure cookers,” saying the technique was “commonly taught in Afghan terrorist training camps” and also had been used in India.
“Typically, these bombs are made by placing TNT or other explosives in a pressure cooker and attaching a blasting cap at the top of the pressure cooker,” says the report, which is available online. “The size of the blast depends on the size of the pressure cooker and the amount of explosive.”
These types of devices can be initiated using simple electronic components such as “digital watches, garage door openers, cell phones or pagers,” the warning said.
A July 1, 2010, an unclassified joint Federal Bureau of Investigation-Homeland Security Department advisory for law enforcement agencies said such bombs have been used in Afghanistan, India, Nepal and Pakistan.
Al-Qaeda promoted the technique in the article “Make a bomb in the kitchen of your mom” by the “AQ chef,” which appeared in the summer 2010 issue of Inspire, the terrorist group’s online English-language magazine.
The two Americans behind Inspire, al-Qaeda cleric Anwar al-Awlaki and propagandist Samir Khan, were reported killed in Yemen by U.S. Predator drones in September 2011. The eight-page guide, complete with step-by-step illustrations, can still be found on the Internet.
In July 2011, Army Private First Class Naser Jason Abdo, a convert to Islam who had refused to serve in Afghanistan citing his faith, was arrested and charged with planning a terrorist attack on soldiers near Fort Hood, Texas. He had the Inspire article, two pressure cookers, six bottles of smokeless gunpowder, and other bomb-making materials, according to an FBI affidavit. He was convicted and given a life sentence last August.
Two U.S. intelligence officials, though, said the instructions are available elsewhere online and the presence on Islamic extremist websites doesn’t indicate that the Boston bomber was a radical Muslim or even inspired by religious rhetoric. The officials, who have access to classified information, asked that they not be identified.
Assembling even a primitive pressure-cooker bomb takes a degree of expertise, according to Burton.
“I’ve constructed bombs, and it’s easy to screw up,” he said. “Getting it right suggests the individual who put them together had some experience and had either practiced or received training.”
In Boston, authorities at a news conference yesterday urged residents to turn over photos and video of the scene. That will help investigators create a timeline of events, as well identify people who were present. Evidence collected at the scene is geo-located through the Global Positioning System, allowing investigators to photograph the scene and turn it into schematic drawings that can be reproduced.
Forensic examiners will log every piece of evidence, and bombing materials probably will receive a preliminary analysis in Boston before being sent to the FBI laboratories in Quantico, Virginia, Murphy said. Authorities will compare bomb materials to a database of those used in other crimes and overseas in places such as Iraq and Afghanistan, he said.
“Is it an explosive like a powder easily available to the public, or is it something unique and manufactured, and would you have to have expertise?” said Chipman, who investigated the bombing of the Alfred P. Murrah Federal Building in Oklahoma City in 1995 and the first World Trade Center bombing in New York in 1993. He left the ATF last year.
Investigators are also searching for the components that triggered the explosion, such as a timer or a remote detonator, Chipman said.
ATF agents can look at some piece of wire and say well that’s this timer or that timer,’’ said Paul Snabel, a former assistant director of the agency. They keep a reference library of the “debris they’ve accumulated over the years.”
Gene Marquez, the ATF’s acting special agent in charge in Boston, said at a press conference yesterday that it remains too early for the agency to discuss the material used for the explosive. Agents were continuing to scour the scene for evidence, some of which had been found in on the rooftops of nearby buildings.
Analysts will look at tips and intercepted phone calls and jihadist website chatter from recent weeks and months to see if, in hindsight, there’s anything that may be useful in identifying a suspect, Murphy said. They’ll also examine current intercepted communications to determine whether there’s chatter about the attack.
Top investigators told lawmakers “they couldn’t confirm there was any chatter” among foreign-based terrorist groups, Ohio Democrat Marcy Kaptur told reporters after House members were briefed by FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano. “To me it suggests” an act of domestic terrorism, said Kaptur, top Democrat on the defense appropriations subcommittee.
Based on the nature of the attack, Murphy said he suspects it may have been carried out by someone inspired by al-Qaeda or radicalized by reading jihadist magazines or websites. The majority of U.S. terror suspects in recent years have been self-radicalized, he said.