In this undated photo provided by Robin Young, Dzhokhar A. Tsarnaev, poses for a photo after graduating from Cambridge Rindge and Latin High School. Tsarnaev has been identified as the surviving suspect in the marathon bombings. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during a getaway attempt in a long night of violence that left one of them dead and another still at large Friday, April 19, 2013. (AP Photo/Robin Young)The Associated Press
A sign calling for citizens of Boston to "Shelter in Place" is shown on I-93 in Boston Friday, April 18, 2013. Two suspects in the Boston Marathon bombing killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during their getaway attempt in a long night of violence that left one of them dead and another still at large Friday, authorities said as the manhunt intensified for a young man described as a dangerous terrorist. (AP Photo/Elise Amendola)The Associated Press
This undated photo provided by the Middlesex District Attorney's Office shows Massachusetts Institute of Technology Police Officer Sean Collier, 26, of Somerville, Mass., who was shot to death Thursday, April 18, 2013 on the school campus in Cambridge, Mass. Authorities said surveillance tape recorded late Thursday showed one of the Boston Marathon bombing suspects during a robbery of a nearby convenience store before Collier was shot to death while responding to a report of a disturbance. (AP Photo/Middlesex District Attorney's Office)The Associated Press
Two explosions near the finish line of the Boston Marathon on Monday killed three people and wounded scores. A look at the basics of the case:
Boston-area residents are being urged to stay inside and lock their doors as state police go door to door in their search for Suspect No. 2, on the loose after his brother was killed in a getaway attempt. An uncle pleaded on live television: "If you are alive, turn yourself in and ask for forgiveness." Connecticut State Police say the at-large suspect could be traveling in a green Honda Civic.
HOW THE SITUATION UNFOLDED
Surveillance tape late Thursday showed Suspect No. 2 during a robbery of a convenience store. A responding police officer was fatally shot. The two suspects carjacked a man, released him and got involved in a chase with police that resulted in explosives being thrown from the suspects' car and an exchange of gunfire, authorities say. Suspect No. 1 died at a hospital.
Law enforcement officials and family members have identified them as Dzhokhar and Tamerlan Tsarnaev, ethnic Chechen brothers who had lived in Dagestan in southern Russia. They had been in the U.S. for about a decade and lived near Boston, an uncle said. Dzhokhar, for whom police are searching, is a 19-year-old student at the University of Massachusetts at Dartmouth. Tamerlan was a 26-year-old boxer. Their motive remains unclear.
THE MARATHON 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 marathon. An 8-year-old boy, a 29-year-old woman and a 23-year-old graduate student from China were killed, and more than 180 people were wounded. The explosions occurred about 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.
Authorities have said they believe the bombs used were fashioned out of ordinary kitchen pressure cookers packed with explosives, nails, ball bearings and metal shards. They suspect the bombs were hidden in duffel bags and left on the ground.