According to the New York Times, Boston Globe, and just about any other major media outlet, there has been a major explosion at the finish line of the Boston Marathon this afternoon. BloombergTV has a live feed of the scene. From the New York Times:
The Associated Press reported that the authorities were helping injured runners leave the scene and bloody spectators were being carried to a medical tent that was being used for runners.
The A.P. said that a loud explosion was heard on the north side of Boylston Street, near a photo bridge that marks the finish line. Another explosion was heard several seconds later.
Details are slowly emerging, but journalists and witnesses are reporting major injuries and possible casualties on social media. (Warning: Some of the images are graphic.) (Update: the Boston Police Department is reporting two deaths and 23 injuries from the blast. A third explosion was heard just before 4 p.m. President Obama is being briefed, and security is being increased at nearby hotels and “prominent locations in New York.”)