One woman says she was waiting for her husband to cross the finish line, and, in her words, "it just blew." She described it as "a loud boom, and then glass everywhere." Cherie Falgoust says something hit her head, and she "just ducked.
A runner, Laura McLean of Toronto, says she heard two explosions outside the medical tent. She says, "There are people who are really, really bloody." McLean says, "they were pulling them into the medical tent."
The explosions took place about three hours after the winners crossed the finish line. The second one could be heard a few seconds after the first one.
A runner said, "There are a lot of people down."
Marathon workers were seen carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg.
Third explosion at JFK Library
Boston police say there's been a third explosion in the city, following two blasts near the finish line of the Boston Marathon that killed three people and injured many others.
Police Commissioner Edward Davis says authorities aren't certain that the explosion at the JFK Library was related to the other blasts, but they're treating them as if they are.
David says there are no injuries stemming from the third explosion.
He urged people to stay indoors and not congregate in large groups.
Obama: 'We will find out who did this'
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. At least three people were killed and 100 injured in the blasts.
Obama has been in touch with federal law enforcement and Massachusetts officials in the aftermath of the explosions.
The Secret Service reacted cautiously to the blasts, expanding the security perimeter around the White House.
White House response
The White House says President Barack Obama has called Boston's mayor and the Massachusetts governor to express his concern for those injured in the Boston Marathon explosions.
Obama is quoted as telling Mayor Tom Menino and Gov. Deval Patrick that his administration would provide whatever support was needed in responding to the incident.
The president was briefed on the explosions by Homeland Security adviser Lisa Monaco shortly after 3 p.m., Eastern time.
Shortly after the explosions, Secret Service shut down Pennsylvania Avenue outside the White House, cordoning off the area with yellow police tape. Several Secret Service patrol cars also blocked off the entry points to the road.
The White House was not on lockdown and tourists and other onlookers were still able to be in the park across the street from the executive mansion.
Rep. Raul Ruiz statement
Local congressman Raul Ruiz released this statement: "I am deeply saddened by the explosions in Boston. My thoughts and prayers are with the people of Boston and the victims of this terrible tragedy."
No-fly zone created over Boston