Two blasts, at least two unexploded devices found
The first explosion took place around four hours after the marathon began and around two hours after the men's winner cross the line. The second explosion took place seconds later.
The first loud explosion occurred on the north side of Boylston Street, just before the photo bridge that marks the finish line. (Watch the video below or see images here)
Around 17,000 of the runners had finished the race, but thousands of others were farther back along the course. (Here's a interactive map from the Boston Globe)
According to a Wall Street Journal report, counter-terrorism officials have found what they believe to be five additional, undetonated explosive devices around the Boston area. Associated Press reported that two unexploded devices were found.
Casualties due to the blasts
Boston police said three people were killed in the blasts . Hospitals reported at least 144 injured, at least 15 of them critically. The injuries ranged from cuts and bruises to amputations.
Many victims suffered lower leg injuries and shrapnel wounds. Some suffered ruptured eardrums.
A woman who was near the second bomb, Brighid Wall, 35, said that when it exploded, runners and spectators froze, unsure of what to do.
Her husband threw their children to the ground, lay on top of them and another man lay on top of them and said, "Don't get up, don't get up."
How many people were present?
23,000 runners took part in the Marathon, which is one of the of most famous globally, while there were also more than 500,000 spectators.
The marathon is held on Patriots Day, a Massachusetts state holiday which commemorates the first battles of the American Revolution in 1775. An column in the Washington Post captures why by targeting the marathon, terrorists have struck at the crown jewel of sport (Read it here)
A few miles (kilometers) away from the finish line and around the same time, a fire broke out at the John F. Kennedy presidential library. The Boston police commissioner said it may have been caused by an incendiary device but didn't appear to be related to the bombings.
No suspects yet
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 has taken charge of the investigation.
President Barack Obama vowed that those responsible will “feel the full weight of justice.”
“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.”
He, however, declined to answer a question on whether the attacks were acts of terrorism.
A White House official speaking on condition of anonymity because the investigation was still unfolding said the attack was being treated as an act of terrorism.
American cities places on alert
Several mega cities in the US, including New York, Los Angeles, Chicago and Washington, have been put on alert.
Security in and around all critical establishments has been tightened and flight movements over the bomb blast site in Boston were stopped from some time.