A look at the bombing at the Boston Marathon and the search for suspects

Two men in haz-mat suits investigate the scene of the first bombing on Boylston Street in Boston Tuesday, April 16, 2013 near the finish line of the 2013 Boston Marathon, a day after two blasts nearby killed three and injured over 170 people. (AP Photo/Elise Amendola) (The Associated Press)

BOSTON – An explosion at the finish line of the Boston Marathon on Monday killed three people and injured dozens more. 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 170 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 two bombs were fashioned out of 6-liter kitchen pressure cookers, packed with shrapnel and explosives, and hidden in black duffel bags left on the ground, according to a person briefed on the case who spoke on condition of anonymity to discuss the ongoing investigation. No arrests had been made, and police and federal agents renewed appeals for any video, audio and photos taken by marathon spectators.

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THE VICTIMS

The 8-year-old boy killed in the bombings, Martin Richard, was remembered by friends and neighbors as a vivacious boy who loved to run, climb and play sports. Also killed was Krystle Campbell, a 29-year-old restaurant manager from Medford, Mass., whose father, William Campbell, said she gone with a friend to watch the race.

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PRESIDENTIAL RESPONSE

President Barack Obama said that the bombings were an act of terrorism but that investigators do not know whether they were carried out by an international organization, a domestic group or a "malevolent individual." He said: "The American people refuse to be terrorized."

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SECURITY RESPONSE

The area around Copley Square remained closed Tuesday, and security was tight around Boston, with bomb-sniffing dogs checking Amtrak passengers' luggage at South Station and transit police patrolling with rifles. 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.

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WHAT'S NEXT

The FBI, U.S. attorney's office and other law enforcement officials were tentatively planning their next media briefing around 5 p.m. Tuesday. 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.