FBI: Devices appear to have been pressure cookers filled with nails, BBs

Richard DesLauriers, head of the FBI in Boston, says remains of the devices have been sent to an FBI lab in Virginia for analysis.

DesLauriers said the heavy devices appeared to have been carried in black nylon bags or backpacks and asked the public to alert the FBI to anybody they might know who had recently blown things up or asked questions about topics such as explosives, or bought several pressure cookers or asked about April 15. Transcript of his comments.

An ATF official said bits of the explosive devices were recovered from Boylston Street rooftops.

He said no suspect has yet been identified and that the FBI is still going through some 2,000 tips received as of noon today.

Police Commissioner Ed Davis said Boston Police had more officers than ever at the finish line; said protecting such a large event is difficult in part because it's "a soft target."

"You can't lock it down like a military operation," he said.