Three people were killed and more than 100 injured as two blasts brought death and destruction to the finishing line of the Boston Marathon.
Horrifying images of injured spectators and blood-splattered pavements were caught as television cameras broadcast footage of the explosions yesterday.
An eyewitness who was metres from the blast when two explosions detonated close to the finish line of the Boston Marathon said he heard a loud boom and 'everyone stopped':
Participants were also seen lying on the ground as the impact tore through the finish line, sending smoke and debris soaring into the air.
The death toll today rose as the FBI said it was leading a "potential terrorism investigation" by the city's joint terrorism taskforce into the twin explosions.
Warning: This footage contains scenes which may cause distress:
But reports of a suspect being held at a city hospital were dismissed by Ed Davis, Boston's Police Commissioner, who told a press conference this morning that no suspect had been arrested.
In a televised address, US President Barack Obama said those responsible will feel the "full weight of justice".
Speaking from the White House, he paid tribute to those who helped wounded runners and spectators.
Trauma surgeon Pete Fagenholz from Massachusetts General Hospital said he has been in surgery all day and has seen a lot of shrapnel injuries, mostly to people's legs:
A list of competitors on the Boston Marathon website showed hundreds of British runners were expected to line up for the race.
The site lists more than 25,000 participants, 347 of whom are British - although it is unclear whether any were caught in the blasts.
Prime Minister David Cameron led politicians in tweeting their solidarity with those affected, saying: "The scenes from Boston are shocking and horrific - my thoughts are with all those who have been affected.
"Security for the London Marathon on Sunday will be reviewed following the explosions in Boston, the Metropolitan Police said.
An eye witness described the moment that people watching the end of the Boston Marathon realised that a bomb had gone off, saying she felt a 'huge detonation'.Warning: This footage contains scenes which may cause distress:
The Boston marathon - one of the world's oldest - is held each year on Patriot's Day.
Commissioner Davis said there had been two "simultaneous explosions" near the finish line of the 26.2 mile event around four hours into the race.
He told a press conference in the early hours of this morning that "at least three people" were dead but that exact figures on the number injured would be released later today."This cowardly act will not be taken within stride, we will turn every rock over to find the people responsible for this," he said.
Steve Silva, a photographer for Boston.com, was filming runners crossing the finish line as the first explosion went off.
His footage contains graphic images that some may find distressing:
Commissioner Davis earlier said there had also been a third explosion at the JFK Library in the city but he did not believe there were any casualties as a result of that blast.
The explosion is not currently being linked with the two at the race.
Bomb disposal teams carried out a controlled explosion on another suspected device, and Commissioner Davis said a number of unattended packages along the marathon route were being treated with caution.
Massachusetts governor Deval Patrick said that Boston would be open today but that it would not be business as usual, with people using the "T" public transport subject to searches.
The chief executive of the London Marathon said security for Sunday's race will be reviewed with the Met Police in light of the explosions in Boston:
The blasts come just six days before the London Marathon on Sunday, prompting a review of security ahead of the event.
Chief Superintendent Julia Pendry, of the Met Police, the event commander for the race, said security arrangements will be reviewed following the explosions in Massachusetts.
She said: "A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon."
Nick Bitel, the chief executive of London Marathon, said: "We are deeply saddened and shocked by the news from Boston.
"Our immediate thoughts are with the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running.
"Our security plan is developed jointly with the Metropolitan Police, and we were in contact with them as soon as we heard the news."