Two explosions detonated near the finish line of the Boston Marathon on Monday.
Hospitals reported at least 144 people injured, at least 17 of them critically. The victims' injuries included broken bones, shrapnel wounds and ruptured eardrums.
A senior U.S. intelligence official said two other bombs were found near the end of the 26.2-mile course in what appeared to be a well-coordinated attack.
The fiery twin blasts took place about 10 seconds and about 100 yards apart, knocking spectators and at least one runner off their feet, shattering windows and sending dense plumes of smoke rising over the street and through the fluttering national flags lining the course. Blood stained the pavement, and huge shards were missing from window panes as high as three stories.
President Barack Obama, responding to the explosions at the Boston Marathon, says the United States does not know "who did this or why" but vowed that whoever is responsible "will feel the full weight of justice."
He said: "We will find out who did this and we will hold them accountable."
Obama made his remarks Monday evening from the White House about three hours after two explosions detonated near the marathon's finish line.
Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads checked parcels and bags left along the race route.
"Police were basically pleading with everybody to get out of the way," said Curt Nickisch of WBUR.
Video of the explosions:
About two hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.
"Fortunately, the explosions happened very late in the race," Nickisch said. "This was hours after the winners went through and the crowd of runners was thinning out pretty well, so it wasn't as densely packed."
Authorities shed no light on a motive or who may have carried out the bombings, and police said they had no suspects in custody. Authorities in Washington said there was no immediate claim of responsibility. The FBI took charge of the investigation.
A few miles away from the finish line and around the same time, a fire broke out at the John F. Kennedy Library. The police commissioner said it may have been caused by an incendiary device but didn't appear to be related to the bombings.
Competitors and race volunteers were crying as they fled the chaos. Authorities went onto the course to carry away the injured while race stragglers were rerouted away from the smoking site.
Roupen Bastajian, a 35-year-old state trooper from Smithfield, R.I., had just finished the race when they put the heat blanket wrap on him and he heard the first blast.
"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."
A Boston police officer was wheeled from the course with a leg injury that was bleeding.
"There are a lot of people down," said one man, whose bib No. 17528 identified him as Frank Deruyter of North Carolina. He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg.
Smoke rose from the blasts, fluttering through the national flags lining the route of the world's oldest and most prestigious marathon. TV helicopter footage showed blood staining the pavement in the popular shopping and tourist area known as the Back Bay.
Cherie Falgoust was waiting for her husband, who was running the race.
"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."
Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.
Flights out of Boston Logan Airport were briefly put on ground stop.
The Red Cross set up a place for family members and loved ones to connect. Find it at this link.
Google has also stepped in to help family and friends of Boston Marathon runners find their loved ones after explosions near the finish line.
The site, called Google Person Finder, allows users to enter the name of a person they're looking for or enter information about someone who is there.
Cellphone use has been difficult in the Boston area. There are conflicting reports as to whether cell service was shut down or networks were overwhelmed.
A law enforcement official, citing an intelligence briefing, said cellphone service had been shut down Monday in the Boston area to prevent any potential remote detonations of explosives.
But officials with Verizon Wireless and Sprint Nextel said there had been no such requests.
Sprint spokeswoman Crystal Davis said: "Minus some mild call blocking on our Boston network due to increased traffic, our service is operating normally."
Audio from the police scanner at the time of the explosions:
The Associated Press contributed to this report
This story was updated at 3:45 p.m. to update injury numbers and add in President Obama's remarks; 3:09 p.m. to add in information about cellular carriers; and 6:30 p.m. to update injuries.