BOSTON—Rep. Stephen Lynch, a Massachusetts Democrat who sits on a national security subcommittee in the house, has told Yahoo News that many of the injured at Monday's Boston Marathon were hit by ball bearings apparently embedded within the devices.

The Associated Press, citing anonymous law enforcement sources, reported that the bombs were made from pressure cookers stuffed with metal ball bearings. CBS News reported that police have found pieces of an electronic circuit board that they believe could have been used to detonate the pressure cookers. Boston Police Commissioner Ed Davis said at Tuesday morning's press conference that law enforcement swept the marathon area for bombs twice before the explosions went off.

For their part, state and federal investigators have declined to say much about the devices that were detonated, sending 176 people to the hospital and killing three, or who they suspect might be behind them.

Meanwhile, Chief of Emergency Medicine at Brigham Women's Hospital Dr. Ron Walls said in a press conference Tuesday morning that one patient had a carpenter nail embedded in him, while two others had numerous pieces of round, shiny debris in their legs that did not appear to come from the street.

Lynch said the ball bearings led him to believe the whoever launched the attack knew what they were doing.

“I have done post-attack analyses on attacks around the world, trying to learn lessons. ... The timing here, the devices themselves, the ball bearings used—you know, that’s not some kid in his garage putting something together,” Lynch said.

Though he repeatedly said he didn’t want to get ahead of investigators, Lynch disputed speculation that the devices strongly indicated domestic, rather than international terrorism.

“I don’t think the trappings are … hokey or homegrown,” Lynch said. “Having two explosions, 11 seconds apart, that points to someone who has more competence at this and has training.”

Danny Defenbaugh, the retired special agent who led the 1995 Oklahoma City bombing investigation, told Yahoo News that investigators will try to reconstruct the devices to see if they can identify a "signature" that points to a specific group or individual. Pressure cookers were used in both foreign and domestic terror incidents in the past few years, including the attempted Times Square bombing in 2010.

“It was undoubtedly a terrorist attack, however I’d be very careful in trying to subcategorize it as international or domestic,” Defenbaugh said. “History has proven that’s just not wise.”

--Liz Goodwin is reporting from New York.