Federal agents zeroed in Tuesday on how the Boston Marathon bombing was carried out — with kitchen pressure cookers packed with explosives, nails and other lethal shrapnel — but said they still didn't know who did it and why.

An intelligence bulletin issued to law enforcement and released late Tuesday included a picture of a mangled pressure cooker and a torn black bag the FBI said were part of a bomb.

The FBI and other law enforcement agencies repeatedly pleaded for members of the public to come forward with photos, videos or anything suspicious they might have seen or heard.

"The range of suspects and motives remains wide open," Richard DesLauriers, FBI agent in charge in Boston, said at a news conference. He vowed to "go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime."

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."

Scores of victims remained in hospitals, many with grievous injuries, a day after the twin explosions near the marathon's finish line killed three people, wounded more than 170 and reawakened fears of terrorism. 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.

Officials found that the bombs in Boston consisted of explosives put in ordinary 1.6-gallon pressure cookers, one with shards of metal and ball bearings, the other with nails, according to a person close to the investigation who spoke on condition of anonymity because the probe was still going on. The bombs were stuffed into black duffel bags and left on the ground, the person said.

DesLauriers confirmed that investigators had found pieces of black nylon from a bag or backpack and fragments of BBs and nails, possibly contained in a pressure cooker. He said the items were sent to the FBI laboratory at Quantico, Va., for analysis.

The FBI said it is looking at what Boston television station WHDH said are photos sent by a viewer that show the scene right before and after the bombs went off. The photo shows something next to a mailbox that appears to be a bag, but it's unclear what the significance is.

"We're taking a look at hundreds of photos, and that's one of them," FBI spokesman Jason Pack said.

Investigators said they haven't determined what was used to set off the explosives.