BOSTON – 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.

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 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 of the Boston bombing 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.

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 under way.

Both 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 had been 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.

Pressure-cooker explosives have been used in international terrorism, and they 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.

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

He urged people to come forward with anything suspicious, such as having heard someone express an interest in explosives or a desire to attack the marathon, having seen someone carrying a dark heavy bag at the race or having heard mysterious explosions recently.

“Someone knows who did this,” the FBI agent said.

The bombs exploded 10 or more seconds apart, tearing off victims' limbs and spattering streets with blood, turning the festive race into a hellish scene of confusion, horror and heroics.

The blasts killed Martin Richard, 8, of Boston; Krystle Campbell, 29, of Medford, Mass.; and Lu Lingzi, a Chinese national who was a graduate student at Boston University.

Doctors who treated the wounded corroborated reports that the bombs were packed with shrapnel intended to cause mayhem.

Obama plans to visit Boston on Thursday to attend an interfaith service in honor of the victims. He has traveled four times to cities reeling from mass violence, most recently in December after the schoolhouse shooting in Newtown, Conn.

Following 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 throughout Chicago's Union Station.

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

Investigators in the Boston bombing are also 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.

After a briefing by intelligence officials, Sen. Saxby Chambliss, R-Ga., said, “There are a lot of things that are surrounding this that build an indication that it may have been a domestic terrorist.” But neither he nor law enforcement officials cited any specific evidence pointing to a source of the attack.

Chambliss said a Saudi national who was injured in the bombings is considered a witness. The man, who is recuperating at a Boston hospital, is in his 20s and is in the United States on a Saudi scholarship to study in the Boston area. An official at the Saudi Embassy in Washington said the man cooperated with police and gave authorities permission to search his apartment.

Away from the bombing site, Boston sprang back a bit Tuesday with Red Sox Nation resiliency – as much as it could, given that 12 blocks of the city's Back Bay neighborhood remained cordoned off as the most complex crime scene in city history.

Less than 24 hours after the attack, diners sat at tables adorned with white napkins outside Five Napkin Hamburger. At Dartmouth and Stuart streets, the corner Starbucks was offering free coffee and pastries. A few blocks away, city officials delivered 1,500 gear bags that had been waiting at the finish line for runners who didn't get to complete the race.

The Washington Post contributed to this report.