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 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 Barack Obama branded the attack an act of terrorists, but he 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 176 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 the bombs 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 ongoing. Both bombs were stuffed into black duffel bags and left on the ground, the person said.

DesLauriers confirmed 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 in Quantico, Va., for analysis.

Investigators have not determined what was used to detonate the explosives.

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.

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

He urged people to come forward with anything suspicious, such as hearing someone express an interest in explosives or a desire to attack the marathon, seeing someone carrying a dark heavy bag at the race, or hearing 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, instantly turning the festive race into a hellish scene of confusion, horror and heroics.

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

"We've removed BBs and we've removed nails from kids. One of the sickest things for me was just to see nails sticking out of a little girl's body," said Dr. David Mooney, director of the trauma center at Boston Children's Hospital.

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."

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.

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.

One of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon is seen in Boston, Tuesday, April 16, 2013, one day after bomb blasts killed three and injured over 140 people. FBI agents searched a suburban Boston apartment overnight and appealed to the public for amateur video and photos that might yield clues to who carried out the Boston Marathon bombing. (AP Photo/Elise Amendola)

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

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

Investigators also are 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.

Massachusetts Gov. Deval Patrick said no unexploded bombs were found at the marathon. He said the only explosives were the two that went off Monday.

Davis said 176 victims came to hospitals around Boston, and 17 of those are in critical condition. He called it the most complex crime scene in the history of the department.

DesLauriers said there are no known additional threats.

Suffolk County District Attorney Daniel Conley said "what occurred in Boston was an act of cowardice."

Authorities are looking for amateur video and photographic evidence that may hold clues to who set off the bombs.

FBI agents searched a suburban Boston apartment overnight and appealed to the public for amateur video and photos that might yield clues.