Security has been stepped up around the US after twin blasts near the finishing line of the Boston Marathon left three people dead, including an eight-year-old boy, and at least 139 injured.
A White House official said the explosions occurred just before 3pm (18:50 GMT) on Monday and would be handled as an "act of terror". The FBI has taken over the investigation into the attacks.
Police officials said mobile phone service have been shut down in the Boston area to prevent any potential remote detonations of explosives.
A senior US intelligence official said two more explosive devices had been found near the scene of the explosions and were being dismantled.
Ed Davis, the Boston Police Department (BPD) commissioner, said that powerful devices had caused the blasts.
"We are questioning many people but there is no suspect in custody," he said. “The BPD is on high alert. All sworn personnel working around the clock. Twelve hour shifts in effect. Days off cancelled.”
A fire at the John F Kennedy Presidential Library also took place several miles away from the explosions more than an hour later. However, Davis said investigators had not linked the fire to the blasts.
Barack Obama, the US president, said it was unclear who was behind the explosions but vowed to hold accountable whoever carried out the blasts. The president said he had directed his administration to increase security around the country.
| A witness describes the aftermath
"We still don't know who did this or why," Obama said, adding, "Make no mistake: We will get to the bottom of this".
The White House was cordoned off with tape after news of the explosions broke.
New York Mayor Michael Bloomberg said security was being stepped up at "strategic locations and critical infrastructure, including our subways".
"We have 1,000 members of the NYPD assigned to counterterrorism duties, and they, along with the entire NYPD and the investments we have made in counterterrorism infrastructure, are being fully mobilised to protect our city," Bloomberg said in a statement.
"As law enforcement authorities investigate today's explosions in Boston, I ask all New Yorkers to keep the victims and their families in your thoughts and prayers."
The blasts happened about three hours after the winners crossed the line as thousands of runners finished the 117th running of the Boston Marathon, with crowds watching and cheering at the finish.
There was a loud explosion on the north side of Boylston Street, just before the bridge that marks the finish line.
Another explosion could be heard a few seconds later.
Smoke rose from the blasts, fluttering through the national flags lining the route.
A Rhode Island state police officer, who was running in the event, said he saw at least two dozen people with very serious injuries, including missing limbs.
Many of the Injured spectators were carried to a medical tent that had been set up to care for fatigued runners.
Speaking to Al Jazeera, a spokesperson for Tufts Medical Centre said: "We are treating nine patients, five of them are in surgery. Injuries are significant but not life threatening.
"Four of those surgical cases were serious orthopedic trauma to lower legs, the others have shrapnel wounds and ruptured ear drums."
Massachusetts General Hospital said it was treating 22 patients, including eight critical cases, with injuries ranging from cuts and bruises to amputations.
The race is the world's oldest annual marathon, with 28,000 runners representing athletes from every US state and more than 90 nations.
Following the explosions, the stragglers in the 40km trek were rerouted away from the smoking site of the blasts.
There are people who are really, really bloody
Several competitors and race organisers were crying as they fled the chaos.
"There are a lot of people down," said one man, whose running number 17528 identified him as Frank Deruyter.
Laura McLean, a runner from Toronto, Canada, said she heard two explosions outside the medical tent.
"There are people who are really, really bloody," McLean said. "They were pulling them into the medical tent."
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. The race was later abandoned.
Al Jazeera's Josh Rushing, reporting from downtown Boston, said that there were a number of people there with "just a shocked look on their face".
"I think everyone is trying to put the media-pieces back together, and try to figure out how their going to move forward," added Rushing.
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