Authorities were investigating whether two ethnic Chechen brothers suspected in the Boston Marathon acted alone as they prepared to file charges against the surviving suspect.
Following a tense 24 hours for the Boston area, the manhunt ended on Friday with 19-year-old suspect Dzhokhar Tsarnaev cornered, captured and taken into custody.
Here is a timeline of events:
* Thursday, April 18, about 5:10 p.m.
- The FBI releases photographs and video of two men suspected of setting off bombs, made in pressure cookers and packed with ball bearings and nails at the Boston Marathon on Monday, killing three people and injuring 176.
Video footage shows a man known as suspect No. 1 wearing a dark baseball cap. He was later identified as Tamerlan Tsarnaev, 26.
Suspect No. 2, later identified as Tsarnaev's brother, Dzhokhar, 19, was wearing a white cap backwards in the images. The 30-second videos are played repeatedly on national television, and photographs of the suspects are posted online.
* Thursday night at 9:04 p.m.
- Russian-language social networking site VK shows someone last logged out of what appears to be Dzhokhar Tsarnaev's page. The site had been accessed via mobile device.
* Thursday night around 10:20 p.m.
- Media cited authorities as saying the pair staged a robbery at a convenience store at the Massachusetts Institute of Technology campus in Cambridge, Massachusetts, but state police said later they were not the ones who robbed the store.
* 10:30 p.m.
- Police discover MIT campus police officer Sean Collier, 26, shot multiple times in his car. He was taken to Massachusetts General Hospital and pronounced dead. Collier was shot in his car on a Cambridge street corner, Boston Police Chief Edward Davis said, describing the crime as a "vicious assassination."
* Shortly after 10:30 p.m.
- Police say the two brothers hijack a car and abduct its owner for about a half hour, forcing him to withdraw money from an ATM before releasing him at a gas station where a security camera captures an image of someone who appears to be Dzhokhar Tsarnaev. Police track the car's movements using the captive owner's cell phone, and say the two suspects tell him they bombed the marathon, the Watertown police chief told CNN.
Police track the car as it heads to the Boston suburb of Watertown.
* Friday, April 19, about 12:30 a.m.
Police confront the suspects on a Watertown street. The suspects fire guns and hurl improvised explosives including another pressure cooker bomb and several pipe bombs. More than 200 rounds were fired over about five to 10 minutes, Watertown police said. A transit police officer is hurt in the shootout.
Tamerlan Tsarnaev walks toward police, firing until he runs out of ammunition, according to the police chief's account. He is tackled by police officers, who try to handcuff him in the street. Dzhokhar Tsarnaev drives toward the police officers, who scatter. The vehicle hits the older brother, dragging him a short way down the street.
Tamerlan Tsarnaev is transported to Beth Israel Deaconess Medical Center and later pronounced dead.
* Before 1 a.m.
- A huge manhunt is launched for the second suspect and hundreds of police officers and FBI agents descend on Watertown.
* Between 3 and 4 a.m.
- Massachusetts police announce they will conduct a door-to-door search in Watertown. Citizens are warned to stay indoors.
* Around 5:30 a.m.
- Train service in Boston is suspended.
- By 6 a.m., a heavy contingent of police reinforcements arrives in Watertown. At sunrise, they begin multiple sweeps through the streets and enter and search many homes. State police wear fatigues and carry side-arms and semi-automatic rifles. Some teams are accompanied by black armored trucks as they fan across neighborhoods.
* 8 a.m.
- Massachusetts officials announce they have expanded the shelter-in-place recommendations for the entire city of Boston, effectively putting the city in lockdown as they search for Tsarnaev.
* 10:30 a.m.
- Police say they have located a gray Honda with Massachusetts license plates in the Boston area that they believe had been occupied by a bombing suspect.
* Shortly after 10:45 a.m.
- Top security and counterterrorism advisers finish an hour-long briefing with President Barack Obama in the White House Situation Room on developments in the Boston manhunt.
* Throughout the day
- The search goes on, including the help of Black Hawk helicopters. The manhunt covers 60 percent to 70 percent of the search area by afternoon, Massachusetts State Police Colonel Timothy Alben says.
* 6 p.m.
Police lift the stay-indoors request for the Boston area. Mass transit reopens. Suspect remains at large.
* About 7 p.m.
Gunfire erupts in Watertown and police converge on a house where the suspect is believed to be hiding, about half a mile from the scene of the shootout. Police had been tipped off by a resident who believed someone was hiding in a boat covered in plastic wrap in the house's back yard.
After the exchange of gunfire, authorities try to coax the suspect out of the boat. They track the suspect's movements inside the boat using an infrared camera on a helicopter. An FBI hostage response team tries to convince the suspect to surrender but the suspect is non-communicative. A robot is sent in to remove the plastic wrap.
* About 8:45 p.m.
Boston police say they captured the suspect alive and he is in custody.
* 9:45 p.m.
The suspect is in serious condition at a hospital, police say. He was found covered in blood. The boat hiding place had been slightly outside the police search perimeter throughout the day, law enforcement officials say at a news conference.
* About 10 p.m.
President Barack Obama praises Boston law enforcement officers, says he has instructed the FBI, the Department of Homeland Security and the U.S. intelligence community to help with the investigation.
Compiled based on Reuters reporting, official statements from law enforcement and media reports. All times Eastern Daylight Time.
(Reporting by Sarah N. Lynch, Alina Selyukh and Patrick Temple-West in Washington, Tim McLaughlin, Svea Herbst Bayliss, Stephanie Simon and Aaron Pressman in Boston, Jonathan Allen in New York; Editing by Doina Chiacu and Vicki Allen)