President Barack Obama branded the attack an act of terrorism, whether carried out by a solo bomber or group, and the FBI vowed to "go to the ends of the Earth" to find out who did it.
Scores of victims remained in Boston hospitals, many with grievous injuries, a day after the twin explosions near the finish line of the world's oldest and most prestigious marathon killed three people, wounded more than 170 and reawakened fears of terrorism in the U.S. A 9-year-old girl and 10-year-old boy were among 17 victims listed in critical condition.
Officials found that the bombs consisted of explosives put in common 1.6-gallon (6-litre) pressure cookers, one containing shards of metal and ball bearings, the other packed with nails, and both stuffed into duffel bags, said a source close to the investigation who spoke on condition of anonymity because the investigation was still going on.
At a news conference, FBI agent Richard DesLauriers, FBI agent in charge in Boston, 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 for analysis in Virginia.
DesLauriers said that there had been no claim of responsibility for the attack, and that the range of suspects and motives was "wide open."
Throughout the day, he and other law enforcement authorities asked members of the public to come forward with any video or photos from the marathon or anything suspicious they might have witnessed, such as hearing someone express an interest in explosives or a desire to attack the marathon, or seeing someone carrying a dark heavy bag at the race.
"Someone knows who did this," the FBI agent said.
FBI agents searched an apartment in the Boston suburb of Revere. According to one law enforcement official, the tenant had been tackled by a bystander, then police, as he ran from the scene of the explosions. But the man may simply have been running away to protect himself, said the official, who spoke on condition of anonymity because he wasn't authorized to release details of the investigation.
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.
The blasts killed 8-year-old Martin Richard of Boston, 29-year-old Krystle Campbell of Massachusetts, and a third victim whose name has not been released.
Doctors who treated the wounded corroborated reports that the bombs were packed with shrapnel intended to cause mayhem.
"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 centre 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."
In the wake of the attack, security was stepped up around the White House and across the U.S. Police massed at federal buildings and transit centres in Washington, 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 urged Americans "to be vigilant and to listen to directions from state and local officials." But she said there was no evidence the bombings were part of a wider plot.
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.
Investigators said they have not yet determined what was used to set off the Boston explosives. Typically, these bombs have an initiator, switch and explosive charge, according to a 2004 warning from Homeland Security.
"We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice," the FBI's DesLauriers said.
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.
The Pakistani Taliban, which claimed responsibility for the 2010 attempt in Times Square, has denied any part in the Boston Marathon attack.
Al-Qaida's branch in Yemen gave a detailed description of how to make a bomb using a pressure cooker in a 2010 issue of Inspire, its English-language online publication aimed at would-be terrorists acting alone.
In a chapter titled "Make a bomb in the kitchen of your mom," it says "the pressurized cooker is the most effective method" for making a simple bomb, and it provides directions.
Naser Jason Abdo, a former U.S. soldier, was sentenced to life in prison last year after being convicted of planning to use a pair of bombs made from pressure cookers in an attack on a Texas restaurant frequented by soldiers from nearby Fort Hood. He was found with the Inspire article.
Investigators are combing surveillance tapes and pictures from the Boston attack and appealing to the public to turn over personal photos and video that might yield clues.
"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.
Obama said officials do not know who carried out the attack or why — "whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual."
But he said "any time bombs are used to target innocent civilians, it is an act of terror." And he declared: "The American people refuse to be terrorized."
Obama plans to travel to Boston on Thursday for an interfaith service, Massachusetts Gov. Deval Patrick said.
Sullivan reported from Washington. Associated Press writers Steve LeBlanc, Bridget Murphy, Rodrique Ngowi and Meghan Barr in Boston; Julie Pace and Lara Jakes in Washington; Paisley Dodds in London; Lee Keath in Cario; and Marilynn Marchione in Milwaukee contributed to this report along with researcher Randy Herschaft in New York.