WASHINGTON — 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.