Update, 6:14 p.m.: President Obama delivered a brief address from the White House, emphasizing how much is still unknown. "America will say a prayer for Boston tonight," he began.
"We still do not know who did this or why. And people should not jump to conclusions before we have all the facts," Obama said. He did not call the explosions a "terrorist attack."
Update, 6:02 p.m.: The Boston Police commissioner has said that the JFK Library fire does not appear to have been related to the marathon explosions.
Update, 5:56 p.m.: Flights are now departing and arriving to Logan Airport, and subway service in Boston will resume within the half-hour. The AP has corrected an earlier report claiming that cell service had been shut down to prevent remote detonations; there is and has been cell service in Boston.
Update, 5:29 p.m.: There was in fact no explosion at the Kennedy Library as stated earlier in the day, the Boston Police Department has announced.
Update, 5:05 p.m.: The AP has reported that cell phone service is shut down in central Boston to prohibit the detonation of remote bombs.
Update, 4:54 p.m.: A spokesman for the Boston Police announced that the fire at Boston's JFK Library was caused by a third explosion, which is being treated as related. The Boston Globe has released an updated injury total:
MARATHON EXPLOSION: Toll rises to 2 dead, 64 wounded in Boston Marathon explosions.— Boston Globe News (@GlobeMetro) April 15, 2013
The FAA has announced that all flights in and out of Boston will be grounded this afternoon.
Update, 4:37 p.m.: The AP reports that two additional undetonated explosive devices have been found at the Boston Marathon and are being dismantled.
Update, 4:20 p.m. NBC is citing law enforcement officials who say the explosions were caused by multiple homemade explosive devices.
Update, 4:17 p.m. Several other U.S. cities are now in a state of heightened alert following the explosions earlier this afternoon. Los Angeles County has activated its Emergency Operations Center; Reuters reports that the New York Police Department has stepped up security around landmarks in Manhattan, including near prominent hotels. In Washington D.C., areas around the White House have been cleared of pedestrian traffic.
Update, 4:10 p.m:
Update, 3:58 p.m.: MSNBC is reporting a third, controlled explosion, which Boston.com says was part of bomb squad activities.
Boston Globe reporter Billy Baker tweets that eyewitness accounts from the earlier two blasts are very disturbing:
Now getting gruesome first-hand accounts of hair on fire, severed limbs, battlefield scene in front of Charlesmark Hotel.— Billy Baker (@billy_baker) April 15, 2013
MSNBC reports that according to eyewitnesses, the explosions came from two abandoned packages left on the street. "At first it almost sounded like a cannon blast but it felt so strong that it almost blew my hat off," said one witness. "People were very badly hurt."
Update, 3:45 p.m.: Below, via a local news affiliate, a video of one of the explosions:
Update, 3:35 p.m.: The Mass. Ave Bridge is closed in both directions; Green line service is suspended. CBS reported that the explosions could be heard from inside the Metro tunnels. Fox has a live stream with coverage from the scene of the explosions.
The Boston Globe reports that "at least dozens" of people appeared to be seriously injured.
BREAKING NEWS: At least dozens of people have been seriously injured, a Globe staffer on the scene says.— The Boston Globe (@BostonGlobe) April 15, 2013
Update, 3:28 p.m.: The AP reports that the explosions occurred about three hours after the start of the race, and that medical tents set up to receive fatigued runners are receiving injured spectators and runners:
"There are a lot of people down," said one man, whose bib No. 17528 identified him as Frank Deruyter of North Carolina. He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg. A Boston police officer was wheeled from the course with a leg injury that was bleeding.
About three hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another thunderous explosion could be heard a few seconds later.
Original post, 3:17 p.m.: Dozens of people appear to be injured after explosions went off at the finish line of the Boston marathon this afternoon.
According to CBS, two explosions were heard near the finish line from the Fairmount Copley Plaza Hotel. Streets around Copley Square have been blocked off by police.
Bruce Mendelsohn tweeted this graphic photo from the scene, which shows blood on the sidewalk at the scene of the explosion:
Top image: Boston_to_a_T/Twitter