Guide to the bombing at the Boston Marathon [updated]

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

Update: 12:23pm

A 29-year-old restaurant manager has been identified as one of three people killed in the bombing at the Boston Marathon.

Her father says Krystle Campbell, of Medford, Mass., had gone with her best friend to take a picture of the friend’s boyfriend crossing the finish line on Monday afternoon.

He says the friend was seriously injured in the explosion.

An 8-year-old, Martin Richard of Boston, also died. He was at the finish line watching the race with his family.

Update: 12:23pm

A person briefed on the Boston Marathon investigation says the explosives were in 6-liter pressure cookers and placed in black duffel bags.

The person says the explosives were placed on the ground and contained shards of metal, nails and ball bearings. The person spoke on the condition of anonymity because the investigation was ongoing.

The person says law enforcement officials have some of the bomb components but did not yet know what was used to set off the explosives.

President Barack Obama said Tuesday the bombings were an act of terrorism but investigators do not know if they were carried out by an international or domestic organization, or perhaps by a “malevolent individual.”

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 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.

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 Tuesday 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.