The Boston marathon blasts tore the legs from some of the fittest athletes in America, and forced people who had just run 26 miles to make a final desperate dash in search of safety.
Witnesses have been describing how yesterday's twin explosions, which took place close to the finish line, turned a scene of relief and elation into a war zone as participants and spectators were forced to flee for cover.
One witness, 44-year-old army veteran Bruce Mendelsohn, said: "There was blood smeared in the streets and on the sidewalk", while another, 35-year-old former marine Roupen Bastajian, added: "These runners just finished and they don't have legs now.
"So many of them. There are so many people without legs. It's all blood. There's blood everywhere. You got bones, fragments. It's disgusting."
Bastajian, who was one of the runners in the race, said he and others close to the scene "put tourniquets on. I tied at least five, six legs with tourniquets."
Mark Hagopian, owner of the Charlesmark Hotel, told AFP that he also saw people with their legs blown off, adding: "A person next to me had his legs blown off at the knee -- he was still alive.
"It was bad, it was fast. There was a gigantic explosion... we felt wind on our faces... Police were saying: 'Get out, get out, leave, leave there may be more bombs.'"
'Kind of ironic'
Runners who were exhausted after completing one of the world's most gruelling endurance races were thrown into panic, running in all directions in the aftermath of the explosions.
"We saw two big puffs. I thought maybe it was fireworks. Then it went off again. And then all of a sudden we heard people crying and running away," said Serghino Rene, who was a few blocks away at the time of the blast. "It was a huge horde of people just running away."
Twenty-one-year-old Sarah Joyce, who heard the blasts just after finishing her first marathon said: "It is kind of ironic that you just finished running a marathon and you want to keep running away."
The blasts struck about two hours after the winners had crossed the finish line of the marathon. About 27,000 people were taking part in the event in total.
A report in The New York Post suggests that the police have arrested a 20-year-old suspect, a Saudi Arabian national in connection with the blasts.
According to the report, the suspect is being held at an undisclosed Boston hospital and was apprehended less than two hours after the blasts.
President Barack Obama went on national television to reassure the public that the perpetrators behind the blasts will pay.
"We will get to the bottom of this, and we will find out who did this, we'll find out why they did this," Obama said.
Beefed up security
Following the blasts, Security has been beefed up across the nation and authorities have urged people not to congregate in large crowds, as there could be more blasts.
The Boston blasts come more than a decade after the 11 September 2001 plane attacks on the Twin Towers in New York. About 3,000 people were killed in the attacks.
The Boston Marathon is one of the biggest annual athletic events held in the United States, with tens of thousands of spectators and participants present at the event.