GWEN IFILL: Twenty-seven thousand runners and thousands more spectators had turned out for the Boston Marathon today when terror erupted. Two bombs exploded, and authorities said two people were killed and more than 50 others were wounded.
Within minutes of the blast, wheelchairs and stretchers were ferrying victims up and down Boylston Street, homestretch of the oldest marathon race in the world. Amid the chaos, competitors, race volunteers and spectators ran from the scene in shock.
MAN: And I run over there, and there's body parts. People have been blown apart. They're dead. I mean, there's -- where the window is, the windows are all blown out.
GWEN IFILL: The attack came about three hours after the winners had crossed the finish line. A loud explosion on the north side of the street went off first, followed by a second blast a few seconds later. A number of people were bloodied and had wounded arms and legs.
MAN: They said put him in a wheelchair. We couldn't put him in a wheelchair. There was no way. He was getting really cold.
We were trying to just keep talking to him and keep him alert as possible, as well as the other people laying there. And we wheeled him, you know, on the board, walked him in the stretcher, met us halfway, and took him in the ambulance.
GWEN IFILL: NewsHour production assistant Noreen Nasir was a few blocks away at the time.
NOREEN NASIR, PBS NewsHour: I saw two clouds of smoke go up and heard them -- you know, it was just a matter of seconds in between the two explosions that happened.
And at first, there was confusion. You know, people immediately looked in that direction and started taking photos. And there was a bit of a confusion.
GWEN IFILL: There were several thousand runners still on the course at the time. Marathon organizers sent buses to pick them up.
Meanwhile, police said the motive for the bombings remained unclear. They also reported a third explosion at the John F. Kennedy Library, but it may not be related.
ED DAVIS, Boston Police Commissioner: We have at this point in time determined that there has been a third incident that has occurred. There was an explosion that occurred at the JFK Library. So this is very much an ongoing event at this point in time.
We are not certain that these incidents are related, but we are treating them as if they are. We're recommending to people that they stay home, that if they're in hotels in the area, that they return to their rooms, and that they don't go any place and congregate in large crowds. We want to make sure we completely stabilize the situation.
GWEN IFILL: In the wake of the attack, police in London, Washington, and New York stepped up security.
President Obama was notified. And White House officials said he offered whatever assistance is needed in the response of the ensuing investigation.