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.