A law enforcement official said Monday's bombs were probably detonated by timers. But the FBI said details of the detonating system were unknown.

While the clues moved the investigation forward, they did not make it immediately apparent whether the attack was an act of domestic or foreign terrorism.

"If your experience and your expertise is Middle East terrorism, it has the hallmarks of al Qaeda or a Middle East group," former FBI Assistant Director Tom Fuentes said. "If your experience is domestic groups and bombings that have occurred here, it has the hallmarks of a domestic terrorist like Eric Rudolph in the 1996 Atlanta Summer Olympics bombings."

Fuentes said he has investigated both types of terrorism -- from Iraq to the United States -- and finds the Boston attack has elements of both. "It has the hallmarks of both domestic and international (attacks), and you can see either side of that."