The forensic investigation of the Boston Marathon bombing is developing a preliminary picture of how it was done, though not yet who may 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 the use of improvised explosive devices — at least one a metal pressure cooker packed with explosives — that were left on the ground near the race finish line.
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.
"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 in a phone interview. "There will be traces there."
Investigators can identify the chemical composition of the explosives quickly, even before residue samples are sent to the FBI's laboratory in Quantico, Va., he said. 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."
Law enforcement officials investigating the two explosions think at least one of the devices used a metal pressure cooker and was hidden in a backpack or duffel bag, according to two law enforcement officials briefed on the matter who asked for anonymity to discuss details.
The improvised bombs contained shards of metal, nails, and ball bearings to increase the carnage among race participants and spectators.
Some victims had 40 or more fragments of pellet and nail-like shrapnel embedded in their bodies, said Dr. 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 news briefing Tuesday.
The use of a metal pressure cooker — a variant on the more common pipe bomb — increases the lethality of the blast due to the metal shards. That type of bomb has joined the list of improvised explosive devices in recent years.
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 placed inside. Pressure cooker bombs are made with readily available materials and can be as simple or as complex as the builder decides. These types of devices can be initiated using simple electronic components including, but not limited to, digital watches, garage door openers, cell phones or pagers."
A July 1, 2010, unclassified joint FBI-Homeland Security Department advisory for law enforcement agencies said such bombs have been used in Afghanistan, India, Nepal and Pakistan.
Assembling even that sort of primitive bomb takes a degree of expertise, Burton said.
"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 press conference Tuesday urged residents to turn over photos and video of the scene. That will help investigators will create a timeline of events, as well to identify people who were present.
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 for more detailed analysis, Murphy said.
Government forensics investigators will compare the chemical traces from the devices with information in a large database of explosive signatures. Authorities will compare bomb material to a database of those used in other crimes and overseas in places such as Iraq and Afghanistan, as well as Internet postings and jihadist magazines that suggest how to create explosive devices, Murphy said.
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.
Based on the nature of the attack, Murphy said he suspects it may have been carried out by someone inspired by al-Qaida or radicalized by reading jihadist magazines or websites.
"It's potentially a self-radicalized individual," Murphy said. "The individual gets worked up and has a cause and figures he needs to act out."
The majority of U.S. terror suspects in recent years have been self-radicalized, he said.
With assistance from Michael Riley and Justin Blum in Washington.