A series of explosions erupted near the finish line at the Boston Marathon on Monday. The Bosto…n Police Department confirmed that they were looking into an explosion, but had no further comment.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.Local television showed ambulances at the scene. Pictures posted online showed several injured runners being attended to and smoke around the finish line.The headquarters for the organizers of the marathon, one of the world’s oldest, was reportedly locked down while authorities investigate. Reporters inside the Copley Plaza Hotel, where the media center is and where many elite athletes are staying, were unable to leave.The explosions went off more than four hours after the start of the men’s race, which meant that there were still several thousand runners yet to finish the race.Big city authorities are typically on the highest levels of alert for events like a marathon, said Anthony Roman, a security expert.