BOSTON — 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.

The bombs exploded 10 or more seconds apart, tearing off victims’ limbs and spattering streets with blood. The blasts near the finish line instantly turned the festive race into a hellish scene of confusion, horror and heroics.

The blasts killed 8-year-old Martin Richard, of Boston, and 29-year-old Krystle Campbell, of Medford. The Shenyang Evening News, a state-run Chinese newspaper, identified the third victim as Lu Lingzi. She was a graduate student at Boston University.

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.

Both bombs were stuffed into black bags and left on the ground, the person said.

At Massachusetts General Hospital, all four amputations performed there were above the knee, with no hope of saving more of the legs, said Dr. George Velmahos, chief of trauma surgery.

“It wasn’t a hard decision to make,” he said. “We just completed the ugly job that the bomb did.”

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.

Investigators said they have not yet determined what was used to set off the explosives.

DesLauriers said there had been no claim of responsibility for the attack.

In the wake of the attack, security was stepped up around the White House and across the country. Police massed at federal buildings and transit centers in the nation’s capital, critical response teams deployed in New York City, and security officers with bomb-sniffing dogs spread through Chicago’s Union Station.

Homeland Security Secretary Janet Napolitano said that the stepped-up security was a precaution and that there was no evidence the bombings were part of a wider plot.

Pressure-cooker explosives have been used in international terrorism, and have been recommended for lone-wolf operatives by Al-Qaida’s branch in Yemen.

But information on how to make the bombs is readily found online, and U.S. officials said Americans should not rush to judgment in linking the attack to overseas terrorists.

Pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 intelligence report by the FBI and the Homeland Security Department. One of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the report said.

“Placed carefully, such devices provide little or no indication of an impending attack,” the report said.

The Pakistani Taliban, which claimed responsibility for the 2010 attempt in Times Square, has denied any part in the Boston Marathon attack.

Investigators in the Boston bombing were combing surveillance tapes from businesses around the finish line and asking travelers at Logan Airport to share any photos or video that might help.

“This is probably one of the most photographed areas in the country yesterday,” said Boston Police Commissioner Edward Davis. He said two security sweeps of the marathon route had been conducted before the bombing.

Boston police and firefighter unions announced a $50,000 reward for information leading to arrests.