Boston bomb suspect said to be identified in video

BOSTON (REUTERS) - Investigators of the Boston Marathon bombings believe they have identified a suspect from security video taken before Monday's blasts killed three people and injured 176 others, a US law enforcement source said on Wednesday.

The source said an official announcement was expected later on Wednesday in what is the first major publicly disclosed break in the investigation.

Investigators of the bombings searched thousands of pieces of evidence from cell phone pictures to shrapnel shards pulled from victims' legs.

Based on shards of metal, fabric, wires and a battery recovered at the scene, the focus turned to whoever may have made bombs in pressure cooker pots and taken them in heavy black nylon bags to the finish line of the world-famous race watched by crowds of spectators.

A stretch of Boston's Boylston Street almost a mile long and blocks around it remained closed as investigators searched for clues in the worst attack on US soil since the hijacked plane strikes of Sept 11, 2001.

Cities across the US were on edge after Monday's blasts in Boston. Adding to the nervousness was the announcement that mail containing a suspicious substance addressed to a lawmaker and to President Barack Obama. The FBI said, however, that agents had found no link the attack in Boston.

The blasts at the finish line of Monday's race injured 176 people and killed three: an 8-year old boy, Martin Richard, a 29-year-old woman, Krystle Campbell and a Boston University graduate student who was a Chinese citizen.

Boston University identified the student as Lu Lingzi.

No one has claimed responsibility for the attack.

"Whether it's homegrown, or foreign, we just don't know yet.

And so I'm not going to contribute to any speculation on that," said US  Secretary of State John Kerry, who until January was Massachusetts' senior senator. "It's just hard to believe that a Patriots' Day holiday, which is normally such time of festivities, turned into bloody mayhem."

The FBI was leading the investigation and asking witnesses to submit any photos of the blast site - which was crowded with tens of thousands of spectators, race staff and volunteers and runners. Many of them have turned in thousands of images, authorities said.