A bomber may have been seen amid the Boston Marathon revelers carrying an unusually heavy nylon bag, weighed down with shrapnel-packed explosives, the FBI has suggested. Or perhaps someone heard something beforehand as a culprit tested explosives or expressed an interest in attacking the race.

Law enforcement agencies pleaded Tuesday for the public to come forward with photos, videos or any information that might help them solve the twin bombings that killed three people and wounded more than 170 a day earlier. Investigators circulated information about the bombs, which involved kitchen pressure cookers packed with explosives, nails and other lethal shrapnel — but the FBI said nobody had claimed responsibility.

“Someone knows who did this,” Richard DesLauriers, FBI agent in charge in Boston, said at a news conference where he detailed the type of clues a bomber might have left. “Importantly, the person who did this is someone’s friend, neighbor, co-worker or relative.”

President Barack Obama branded the attack an act of terrorism but said officials don’t know “whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual.” Obama plans to attend an interfaith service Thursday in the victims’ honor in Boston.

Scores of victims of the Boston bombing remained in hospitals, many with grievous injuries. Doctors who treated the wounded corroborated reports that the bombs were packed with shrapnel intended to cause mayhem. A 9-year-old girl and 10-year-old boy were among 17 victims listed in critical condition.

Heightening jitters in Washington, where security already had been tightened after the bombing, a letter addressed to a senator and poisoned with ricin or a similarly toxic substance was intercepted at a mail facility outside the capital, lawmakers said.

There was no immediate indication the episode was related to the Boston attack. Senate Majority Leader Harry Reid said the letter was sent to Republican Sen. Roger Wicker of Mississippi.

In the Boston case, an intelligence bulletin issued to law enforcement includes a picture of a mangled pressure cooker and a torn black bag that the FBI said were part of a bomb that exploded during the marathon.

DesLauriers said cooperation from the community will play a key role in the investigation. He said the range of suspects remained wide open, but by midday Tuesday more than 2,000 tips had been received.

From: http://feeds.foxnews.com/~r/foxnews/national/~3/8AwY9pcM0Go/