At least three people have been killed and more than 140 others injured from two bomb explosions near the finish line of the Boston Marathon.
Police said the blasts were caused by "powerful" devices.
The blasts, which occurred seconds apart on April 15, left streets covered in debris and blood. The blasts tore limbs off people, knocked people to the ground, shattered windows and sent up plumes of smoke.
The victims include children.
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 there were no suspects in custody.
President Barack Obama vowed that authorities will find out who was responsible for the bombing and bring them to justice.
Speaking three hours after the explosions, Obama did not describe the blasts as terrorism, saying investigators were still searching for facts.
“We still do not know who did this, or why, and people shouldn’t jump to conclusions before we have all the facts," Obama said. "But make no mistake: we will get to the bottom of this, and we will find out who did this, we’ll find out why they did this. Any responsible individuals, any responsible groups, will feel the full weight of justice.”
Obama added that authorities will increase security around the United States "as necessary."
Obama has ordered his administration to provide whatever assistance is needed to aid victims and help investigators probe the explosions.
The Federal Aviation Administration has imposed a no-fly zone over the site of the explosions.
Security has also been increased in New York City, Washington, Los Angeles, London and other cities in response to the Boston bombs.
Some reports said at least one unexploded suspected device had been found in Boston and was being dismantled by experts.
Boston Police Commissioner Ed Davis said of the blasts: "These were powerful devices that resulted in serious injury."
"At 2:50 p.m. today, there were simultaneous explosions that occurred along the route of the Boston Marathon near the finish line," Davis said. "These explosions occurred 50 to 100 yards apart, and each scene resulted in multiple casualties."
Davis said police regarded the incident as “ongoing,” and advised people to stay off the streets.
More than 20,000 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, a Massachusetts state holiday commemorating battles of the U.S. war for independence.
With reports from AP, Reuters and AFP