U.S. investigators led by the Federal Bureau of Investigation (FBI) are examining video footage taken by spectators as part of the hunt for who carried out the double-bombing at the Boston Marathon that killed at least three people and injured more than 140 others.
At least a dozen of the injured are said to be in critical condition.
The blasts occurred around 13 seconds apart near the finish line of the marathon, which is run on Patriots’ Day, a Massachusetts state holiday commemorating battles of the U.S. war for independence.
The April 15 explosions tore off limbs, knocked people to the ground, shattered windows, and sent up plumes of smoke.
Children are among the victims, including an 8-year-old who was reportedly among the dead.
The blasts occurred as runners were still racing in America's most famous marathon and thousands of spectators were in the area.
There has been no claim of responsibility, and police said no suspects were in custody. Officials said there had been no credible warnings or threats of a possible attack.
WATCH: "The Boston Globe" posted this video from the scene as police, emergency crews, and bystanders tried to help the victims of the April 15 bombing:
President Barack Obama, who stopped short of describing the incident as "terrorism" and cautioned against "jump[ing] to conclusions," vowed that authorities would find and punish those responsible.
Obama said the investigation was continuing, adding, "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."
David Abel, a reporter from "The Boston Globe,” was close to one of the explosions: “I was standing on the finish line taking video of runners coming in when, about 10 feet [3 meters] from me on the sidewalk, I heard a massive explosion and I saw a white plume of smoke rise up. When I opened my eyes and realized what was going on, I saw a large crowd of people in what was clearly the worst thing I’ve ever seen in my life. There were people lying on the ground, mangled, blood, glass shattered; it was really chaos.”
The Federal Aviation Administration imposed a no-fly zone over the site of the explosions, while security was increased in New York City, Washington, Los Angeles, San Francisco, and other cities in response to the Boston bombs.
Some reports suggested that at least one unexploded suspected device had been found in Boston and was dismantled by experts.
Police have cordoned off the area around the blasts, in the heart of Boston’s shopping district, as investigators scour the zone for clues.
Massachusetts Governor Deval Patrick said within hours of the blasts that "the city of Boston is open and will be open tomorrow, but it will not be business as usual."
Obama said he had ordered that the full resources of the federal government be made available to aid victims and assist in the investigation.
"Boston is a tough and resilient town, so are its people," Obama said. "I’m supremely confident that Bostonians will pull together, take care of each other, and move forward as one proud city. And as they do, the American people will be with them every single step of the way."
FBI Special Agent Richard DesLauriers said the bombings were being treated as a “potential terrorist investigation.”
He said the FBI was leading the probe, with help from Boston and Massachusetts authorities.
Some 23,000 runners from dozens of countries took part in the 42-kilometer race, which is considered the world’s oldest annual marathon.
This year’s edition of the marathon was held in honor of the 26 people who died in the gun massacre in Newtown, Connecticut, in December.
The race is held on Patriots’ Day, fueling speculation that it might have been a radical domestic antigovernment group.
Based on reporting by AFP, AP, and Reuters