The blasts took place in a crowd of spectators, just feet away from the finish line where hundreds of runners were completing the world's oldest annual marathon. Photographs showed the area along Boylston Street covered in injured people, with security guards and emergency workers scrambling to give first aid.
Witnesses said they had seen victims who had lost limbs. "There were a lot of people down," said Frank Deruyter, who was running the marathon.
An eight-year-old was among the dead. At the city's Children's Hospital a nine-year-old girl, a seven-year-old boy, a 12-year-old and another child aged two were among the injured, according to the Globe.
Early on Tuesday morning the Guardian witnessed FBI investigators entering and leaving a building in the Boston suburb of Revere, at one stage taking away a black plastic bag. The Associated Press said Massachusetts state police had confirmed that a search warrant related to the investigation into the explosions was served on Monday night in Revere but authorities provided no further details.
As many as two unexploded bombs were also found near the end of the 26.2-mile (42km) course as part of what appeared to be a well co-ordinated attack but they were safely disarmed, a senior US intelligence official told the Associated Press on condition of anonymity.
The two devices that caused the carnage had detonated without warning at about 2.50pm ET on Tuesday, the Boston police commissioner Ed Davis told reporters at a media briefing. Asked whether the city was under a terrorist attack, he replied: "We're not being definitive about this right now, but you can reach your own conclusions based on what happened."
Early on Tuesday morning the Reuters news agency said the devices used gunpowder as the explosive and were packed with ball bearings and other shrapnel to maximise injuries. Reuters said the description came from a senior law enforcement official briefed on the investigation who declined to be identified because of the sensitivity of the information.
Dozens of injured people were taken to local hospitals where some remained in a critical condition on Monday evening. The Associated Press put the number of injured at more than 130.
Police wearing camouflage uniforms and carrying assault rifles guarded the main entrance of Massachusetts General Hospital on Monday evening. Inside were 29 people injured in the blasts, including eight who were in a critical condition and undergoing surgery. Dr Alasdair Conn, the hospital's chief of emergency services, said at least four of them had arrived with traumatic amputations, meaning their limbs had been blown off.
President Obama, at a briefing at the White House, said: "The American people will say a prayer for Boston tonight. Michelle and I send our deepest thoughts and prayers to the families of the victims in the wake of this senseless loss."
Obama said he had spoken with the FBI director and the Department of Homeland Security, who were co-ordinating the federal response. He stopped short of using the word "terrorism" to describe the explosions but vowed to bring the perpetrators to justice.
"We will find out who did this and we will hold them accountable," he said. "Make no mistake, we will find out who did this and why they did this, and the groups or individuals responsible will feel the full weight of justice."
A White House official, speaking off the record, said the attack was being treated as an act of terrorism. But at a press conference on Monday night the FBI special agent who has taken control of the inquiry, Rick DesLauriers, said it was a "criminal investigation and potential terrorism investigation".
He refused to comment on reports that a "person of interest" was being treated at Brigham and Women's hospital in the city. The police commissioner flatly denied reports there was a suspect at the hospital.
Davis described the loss of life as "horrendous", adding: "This cowardly act will not be taken in stride. We will turn every rock over to find the people who are responsible for this."
The Massachusetts governor, Deval Patrick, said the city would be open on Tuesday "but it will not be business as usual".
- TheGuardian
