Twin bombs exploded at the finish line of the Boston Marathon earlier this week, killing three people and wounding more than 170 others in what police later described as the worst attack in the history of the city. Here is a timeline of the events as they unfolded:
MONDAY, APRIL 15
U.S. & World
2:50 p.m.: Two bombs explode about 20 seconds apart at the finish line of the Boston Marathon, killing three people and wounding more than 170 others. Authorities later said the bombs were filled with nails, BBs and ball bearings, and made from pressure cookers. Many of the wounded were severely injured by shrapnel, and local hospitals reported performing at least a dozen amputations.
THURSDAY, APRIL 18
5:30 p.m.: The FBI releases surveillance video and photos of two suspects wanted in connection with the Marathon bombings. Both are seen walking with backpacks, and suspect No. 2, identified as the man in the white hat, was seen setting a backpack down at the second blast site just prior to the explosion. Sources later told NBC News they also had footage, not to be released, of that suspect watching the first blast and walking away after planting the second bag.
Just before 10:20 p.m.: An armed robbery is committed at a 7-Eleven near MIT, and police initially believe the suspects were the robbers. They later say on Friday that the suspects were not the robbers.
10:20 p.m.: Police receive reports of shots fired on the MIT campus. Responding officers find an MIT campus police officer shot to death in his cruiser, allegedly by the two bombing suspects. Authorities said he had been shot multiple times.
11:30 p.m.: Authorities say two men hijacked a Mercedes SUV at gunpoint in Cambridge and kept the driver hostage for half an hour. He was released at a gas station by midnight and not injured. Authorities begin looking for the stolen vehicle.
FRIDAY, APRIL 19
1 a.m.: Authorities find and pursue the SUV, and the chase ends in Watertown. The men in the vehicle, who authorities determined to be the suspects seen in the surveillance video, begin throwing explosives at law enforcement officers and engage in a shootout with police. One suspect is wounded by police; an improvised explosive device is found strapped to his chest when police approach him. He dies at the hospital a short time later. The second suspect, the man described as wearing the white hat in surveillance video, escapes on foot. Authorities say a Boston transit officer is seriously injured in the shootout.
1:15 a.m.: Federal, state and local law enforcement descend on Watertown and a manhunt is underway for the second suspect. Authorities also begin detonating multiple explosive devices found scattered in the street that the suspects threw from the SUV.
6:45 a.m.: Authorities tell NBC News the suspects are brothers, 19-year-old Dzhokhar Tsarnaev, and 26-year-old Tamerlan Tsarnaev. The younger suspect, Dzhokhar, is the one on the loose.
8 a.m.: Gov. Deval Patrick urges everyone in the entire city of Boston to stay indoors. Authorities later say additional accomplices may also be involved.
11:30 a.m.: The uncle of the two suspects, Ruslan Tsarni, tells reporters in Maryland that his nephews have brought shame on the family and on Chechens. "Turn yourself in," he urges the younger one.
6:10 p.m.: Gov. Patrick says the order for Boston residents to stay home has been lifted, and that subway service has returned. Authorities say they didn't turn up any new information on the day-long search of Watertown. Mayor Menino thanks citizens for their cooperation, saying "Boston will stand tall, we'll never stand down."
7 p.m. Gunfire erupts in Watertown, the site of the overnight shooting that left the first suspect dead. Federal investigators are told a person or a body had been found in a trailered boat in a woman's yard, NBC News reports.
8:45 p.m.: Officials say they have the suspect in custody. Cheers erupt in Watertown.