BOSTON -- Federal agents zeroed in Tuesday on how the Boston Marathon bombing was carried out -- with pressure cookers packed with explosives, nails and other lethal shrapnel -- but said they still didn't know who did it or why.
The FBI and other law-enforcement agencies repeatedly appealed to the public to come forward with photos, videos or anything suspicious they might have seen or heard.
U.S. 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 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 nine-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, six-litre 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 duffel bags and left on the ground.
"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."
DesLauriers confirmed investigators 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 lab in Virginia for analysis.
Investigators said they have not yet determined what was used to set off the Boston explosives.
Homemade bombs built from pressure cookers have been a frequent weapon of militants in Afghanistan, India and Pakistan. Al-Qaida's branch in Yemen once published an online manual on how to make one, urging "lone jihadis" to act on their own to carry out attacks.
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 simple bomb design could imply the maker is an amateur, incapable of acquiring more sophisticated materials, veteran investigators and forensics experts said. But they said it also could be the work of a sophisticated bomb-maker taking great care to cover his tracks.
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 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.
"We found BBs inside of kids. We found nails that looked almost like carpet tacks, maybe a centimetre long, that were sticking out of a kid's body."
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."
"You can't put into words how disturbing this is," said Tracey Dechert, a trauma surgeon who had finished a 28-hour shift at Boston Medical before the twin bombs went off, then raced back to the hospital, arriving in time to perform a double amputation on one woman.
The hospital had five patients who required amputation of part of at least one leg, and "more than one" patient lost both legs, Dechert said.
Obama plans to visit Boston Thursday to attend an interfaith service in honour of the victims. He has travelled 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 U.S. Police massed at federal buildings and transit centres 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.
-- The Associated Press, with files from the Washington Post
Hero in cowboy hat
has endured tragedy
MIAMI -- One of the most searing and iconic images of Monday's bomb blasts in Boston is a long-haired man wearing a cowboy hat, comforting victim Jeff Bauman, who lost his legs in one of the explosions.
The man in the hat is Carlos Arredondo, and he is being heralded as a hero. Ironically, he has spent most of the last decade grieving over his son's death serving the U.S. in the fight against terrorism.
Alex Arredondo, 20, was killed in Iraq in 2004. When the military team arrived at Arredondo's home in Hollywood, Fla., to deliver the news, the Costa Rica native, devastated, torched the military van with a can of gas. He was hospitalized for months with second-degree burns.
The incident made national headlines and spurred a debate over whether he should be charged with a crime. (He wasn't.)
He recovered and became a peace activist and grieved publicly over the loss of his son by displaying makeshift memorials, a mobile flag-draped coffin bearing the uniform, dog tags and Purple Heart of his son.
"As long as there are marines fighting and dying in Iraq, I'm going to share my mourning with the American people," he told the New York Times six years ago.
In the years since, Arredondo, now 52, continued working as a peace activist, travelling around the country, organizing protests and supporting other activists' efforts to end war. Originally from New England, he returned to the Boston area to be closer to his 24-year-old son Brian, who was despondent over the loss of his brother. In December 2011, Brian Arredondo committed suicide.
Arredondo was at the Boston Marathon cheering for a friend who was running in his sons' memory. He was steps away from where the first bomb exploded, witnessing much of the carnage.
His actions mirrored his own son's heroism as Arredondo grabbed a wheelchair and began pushing a victim toward ambulances while holding the man's bleeding leg.
"I kept talking to him. I kept saying, 'Stay with me, stay with me,' " a trembling Arredondo told the Portland Press Herald afterward.
-- Miami Herald