Terrorists reportedly used pressure cookers packed with metal and ball bearings to kill three people at the Boston Marathon, according to a source briefed on the attack.
One of the victims has been named as eight-year-old Martin Richard. More than 170 others, including Martin’s mother and sister, were injured.
Krystle Campbell, 29, of Medford, Massachusetts, was also among the dead, her father William Campbell confirmed on Tuesday night.
US President Barack Obama called the bombing on Monday night a “heinous and cowardly act”. No arrests have been made and it is unclear whether the bombings were carried out by an international organisation, domestic group or a “malevolent individual”, the president said.
A person briefed on the attack said the explosives were in six-litre pressure cookers and placed in black duffel bags on the ground.
The source said the duffel bags contained shards of metal, nails and ball bearings.
Pressure cooker bombs have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 joint FBI and Homeland Security intelligence report.
One of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the intelligence report said.
The Pakistani Taliban, which claimed responsibility for the botched attack on Times Square, has denied any role in the Boston Marathon attack.
The bombs went off on Patriots day, one of Boston’s biggest holidays.
The explosions took place about 10 seconds and about 100 yards apart, knocking over spectators and at least one runner. Victims suffered broken bones, shrapnel wounds and ruptured eardrums.
Martin Richard’s father Bill said in a statement: “My dear son Martin has died from injuries sustained in the attack on Boston. My wife and daughter are both recovering from serious injuries.
“We thank our family and friends, those we know and those we have never met, for their thoughts and prayers. I ask that you continue to pray for my family as we remember Martin.
“We also ask for your patience and for privacy as we work to simultaneously grieve and recover. Thank you.”
Mr Obama said: “Any time bombs are used to target innocent civilians, it is an act of terror,” Obama said. “What we don’t yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organisation – foreign or domestic – or was the act of a malevolent individual.”
He added: “The American people refuse to be terrorised, because what the world saw yesterday in the aftermath of the explosions were stories of heroism and kindness and generosity and love.
“Exhausted runners who kept running to the nearest hospital to give blood and those who stayed to tend to the wounded, some tearing off their own clothes to make tourniquets.”