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US investigators vowed Tuesday to go "to the ends of the Earth" in the hunt for Boston marathon attackers who packed bombs with metal pellets and nails to cause maximum suffering.
US President Barack Obama condemned Monday's attack at the race's finish line, which killed three people and wounded more than 170 others, as "an act of terror."
Obama said there was still no indication as to whether domestic or foreign terrorists carried out the attack and no claim of responsibility was made.
Boston's Boylston Street, scene of the carnage, remained sealed off as investigators sought leads in the worst attack on civilians in the United States since the September 11, 2001 atrocities.
US authorities threw virtually every investigation agency into the hunt. As of Tuesday, no suspects were in custody.
"This will be a worldwide investigation," said Rick DesLauriers, head of the FBI's Boston office.
"We will go to the ends of the Earth to identify the subject or subjects responsible for this despicable crime," he added. "We are using full capacities of the FBI, to its fullest worldwide extent."
New details of the horrific injuries were given by doctors who carried out at least 10 amputations of legs, some at the scene of the blasts.
Investigators quoted by several US media outlets said the bombs were hidden in pressure cookers put in backpacks. Similar easy-to-make roadside bombs are used in Iraq and Afghanistan.
"These bombs contained small metallic fragments more consistent with pellets and other small pieces of metal, but also spiked points that resembled nails without heads," said George Velmahos, head of emergency medicine at Massachusetts General Hospital.
Roger Walls at Brigham and Women's Hospital said a dozen carpentry nails were taken from the body of one victim.
The two bombs, which were 13 seconds and about 100 meters (yards) apart, sprayed the shrapnel into the crowd of thousands of people lining the Boylston Street to watch the runners cross the finish line.
Three people were killed and 176 injured, with 17 people in critical condition, Boston police commissioner Ed Davis told reporters. The dead and injured were aged between two and 71 and included nine children.
Among the dead was an eight-year-old boy, Martin Richard, who had been waiting at the finish for his father to cross the line. His mother suffered grievous brain injuries and his sister lost a leg.
Massachusetts resident Krystle Campbell, 29, was also named as one of the dead.
Liz Norden tearfully told the Boston Globe how two of her sons, both in their 30s, lost legs in the blast.
Most of the 23,000 runners in the 26.2-mile (42-kilometer) race had finished when the first bomb went off, sending out a powerful shockwave.
One 78-year-old runner, Bill Igriff, was blown to the ground and many bloodied spectators were pushed by the concussion through barriers onto the street. Igriff got up again and walked away with just scratches.
Boston relived the horror in the many videos taken with telephone cameras that investigators also pored over in the hunt. Police appealed for the public to send in pictures or videos.
While vigils and other remembrance ceremonies were to be held, armed National Guard troops and police patrolled Boston commuter trains and buses and authorities warned that tight security would be imposed for several days.
Federal and state investigators searched an apartment in the Boston suburb of Revere, and took away bags after a man described as "a person of interest" was stopped.
A 20-year-old Saudi man who suffered a leg injury was initially under armed guard in hospital, but the Saudi embassy in Washington said Tuesday that he was a witness and not accused of any involvement.
"We are interviewing a variety of witnesses right now in a variety of locations," said the FBI's DesLauriers, refusing to give details that could "compromise" the inquiry.
"This was a heinous and cowardly act," Obama said at the White House. "Any time bombs are used to target innocent civilians, it is an act of terror."
"There were no intelligence warnings that we know of," said Representative Peter King, a member of the House Intelligence Committee, who added that investigators had not determined whether foreign or domestic groups were involved.
New York, Los Angeles, San Francisco and several other major US cities boosted security as Russian President Vladimir Putin led global condemnation, describing the twin explosions as "barbaric."
German Chancellor Angela Merkel said "nothing justifies such a malicious attack on people attending a peaceful sporting event." Iran also strongly condemned the blasts.
The national flag over the White House was lowered to half-staff and the New York Stock Exchange held a minute of silence before trading started.
Organizers of Sunday's London Marathon said the race would go ahead, but security arrangements were under review.