Investigators pledged to “go to the ends of the earth” to find whoever was behind the Boston Marathon bombings that took the lives of three people.
Two of those killed have been named as eight-year-old Martin Richard and 29-year-old restaurant manager Krystal Campbell. Martin was killed as he waited with his mother and sister to watch his father cross the line.
President Barack Obama confirmed the FBI was investigating the tragedy as an act of terrorism as it emerged some 170 people were injured in the explosions.
Richard DesLauriers, special agent in charge of the Boston FBI office, said: “We will go to the ends of the earth to find the subject or subjects responsible for this despicable crime.”
There were no immediate claims of responsibility for the attack. The Pakistani Taliban, which has threatened attacks in the US because of its support for the Pakistani government, denied any role.
Police were reportedly searching a flat in the Boston suburb of Revere as part of the investigation, which local media claim is home to a Saudi national injured in the blast.
Mr DesLauriers said: “Our mission is clear to bring to justice those responsible for the marathon bombing.”
The Boston Marathon is one of the world’s oldest and most prestigious and about 23,000 runners took part, including hundreds from the UK and Ireland.
Scores of victims were hurt, many suffering shrapnel wounds, in the two explosions, which were caused by bombs believed to be stashed in bins nearby.
It is also understood the explosive devices were made of pressure cookers packed with metal and ball bearings. Massachusetts General Hospital said “several amputations” had to be performed on survivors.
Horrifying images of injured spectators and blood-spattered pavements were caught as television cameras broadcast footage of the explosions.
Tim Davey, of Richmond, Virginia, was with his wife, Lisa, and children near a medical tent that had been set up to care for fatigued runners when the injured began arriving.
“They just started bringing people in with no limbs,” he said.
The first explosion occurred just before the photo bridge that marks the finish line. The second could be heard a few seconds later.
They happened about four hours into the race, a time when large numbers of runners would be finishing, with many more relatives and friends cheering them on.
London Marathon organisers announced there will be “a period of silence lasting 30 seconds” before the start of the elite men’s race and mass start on Sunday.