An explosion at the finish line of the Boston Marathon Monday killed three people and injured dozens more. Here’s an at-a-glance look at the facts in the case:

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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.

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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.

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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.

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SECURITY RESPONSE

The area around Copley Square remained closed Tuesday morning, as did exit ramps from major highways to the area. The Federal Aviation Administration barred low-flying 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.