"It's like a piano player," Kline said. "You can give Dave Brubeck or Chopin the same piece of music and it will sound different."
With this type of bomb, it can be triggered with something as simple as an egg timer or alarm clock, Parker said. Experts doubt a cellphone was used.
The use of nails, shards of metals and ball bearings also amplifies the personal devastation, experts said.
"We've removed BBs and we've removed nails from kids. One of the sickest things for me was just to see nails sticking out of a little girl's body," said Dr. David Mooney, trauma chief at Boston Children's Hospital, which treated 10 blast victims.
AP writers Alicia A. Caldwell and Eileen Sullivan in Washington contributed to this report.
Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.