BOSTON (AP) — An explosion at the finish line of the Boston Marathon Monday killed three people and injured dozens more. An at-aglance look at the facts in the case:

THE EXPLOSIONS: Two bombs exploded about 10 seconds and 100 yards apart at about 2:50 p.m. Monday in Boston’s Copley Square, near the finish line of the Boston Marathon. Three people were killed, including an 8-year-old boy, and more than 140 were injured. The explosions occurred four hours into the race and two hours after the winners had crossed the finish line, but thousands of runners were still on the course.

THE INVESTIGATION: The FBI took charge of the investigation into the bombings, serving a warrant late Monday on an apartment in the suburban Boston town of Revere and appealing for any video, audio and still images taken by marathon spectators. No arrests had been made, and authorities weren’t commenting about suspects.

PRESIDENTIAL RESPONSE: President Barack Obama vowed that those responsible will “feel the full weight of justice.” The president was careful not to use the words “terrorism” or “terrorist attack” in his remarks, but a White House official, speaking on condition of anonymity because the investigation was still unfolding, said the attack was being treated as an act of terrorism.

SECURITY RESPONSE: The area around Copley Square remained closed this morning, as did exit ramps from major highways to the area. The Federal Aviation Administration barred lowflying aircraft within 3.5 miles of the site.

Other cities also beefed up security in response to the bombing and the Secret Service expanded its security perimeter around the White House.

WHAT’S NEXT: The FBI, U.S. Attorney’s office and other law enforcement officials planned to brief the media at 9:30 a.m. Obama will be briefed today on the investigation and the ongoing response efforts from FBI Director Robert Mueller, homeland security assistant Lisa Monaco and other senior members of his team.