The hunt for the two brothers suspected in the Boston Marathon bombings evolved rapidly Thursday and Friday, ending with one suspect dead and the other captured alive.
Authorities ordered a lockdown of Boston and several surrounding communities Friday after engaging in an overnight battle with the suspects, who police say were armed with guns and explosives. The young men allegedly robbed a convenience store before shooting a campus police officer to death at the Massachusetts Institute of Technology, authorities said.
The pair then reportedly fled in a hijacked SUV, only to get into a firefight with police in the suburb of Watertown. The gun battle left one of the brothers dead. The other escaped and was captured Friday night.
Here's how the violent events transpired, according to authorities and law enforcement sources:
April 18, 5 p.m. (all times ET and approximate) The FBI releases photos and a surveillance video that show two men, one wearing a white baseball cap and the other wearing a black cap. Each carries a backpack in the footage. The FBI says they should be considered "armed and extremely dangerous."
7:10 p.m. NBC News' Pete Williams reports that the FBI has begun receiving names in response to its release of photos.
10:20 p.m. Gunshots are reported on the MIT campus. It is unknown at this time whether or not injuries have occurred. The site is considered "active and extremely dangerous," according to a university alert, and responders cordon off the area.
Around 10:30 p.m. An MIT police officer is found shot in his vehicle. He is taken to Massachusetts General Hospital and pronounced dead. One of the suspects is later seen on a surveillance tape robbing a convenience store in Cambridge, officials tell NBC News. The robbery is believed to have occurred before the MIT officer was killed. Shortly after the shooting, police receive a report of an armed carjacking in a separate section of Cambridge. The carjacking victim was held at gunpoint by two men for half an hour before being released, unharmed, at a gas station on Memorial Drive in Cambridge, according to a statement by the Middlesex District Attorney.
Some time after—it's unclear exactly when—police pursue the carjacked vehicle into Watertown. Explosive devices are reportedly thrown from the SUV, according to the district attorney's statement. A Boston transit police officer is seriously injured. One of the suspects is critically injured and later pronounced dead, according to the statement.
11:20 p.m. Authorities continue to investigate what is now considered an "active shooter incident" on the MIT campus, and people are told to stay indoors and away from the area.
11:41 p.m. An MIT alert says that injuries have been reported in the shooting on campus. "The situation is still very active. and we ask everyone to stay inside."
11:44 p.m. The Massachusetts State Police say in a statement that the search for a suspect or suspects is considered ongoing.
April 19, about 12:15 a.m. A statement from the Middlesex District Attorney says Cambridge police and the state police are investigating a "fatal shooting" in Cambridge, believed to be the MIT officer.
About 1:15 a.m. NBC News' Tom Winter reports a massive police presence, including state troopers and police cruisers with lights and sirens blaring, in the Boston suburb of Watertown, where there have been reports of violence. Several ambulances are also on the scene.
1:31 a.m. MIT advises people on campus to remain indoors, as the police have not determined that the campus is safe.
1:51 a.m. Winter reports that the National Guard and FBI are on the scene in Watertown and have secured a perimeter.
1:57 a.m. "Police have determined that the suspect in this evening's shooting is no longer on campus," MIT tweets. "It is now safe to resume normal activities."
2 a.m. The FBI releases four new photos of the two men, one in a white hat and one in a black hat, at the Boston Marathon.
2:20 a.m. The New York Times publishes an article quoting Watertown residents who say that two men hid behind a black Mercedes SUV and engaged in a shootout with a large number of police officers. The men, about 200 feet apart, exchanged "constant gunfire," and the two shooters lit an explosive; it landed in the space between them and the police, and then exploded. One of the two men then ran toward police and was tackled, an eyewitness tells the Times.
About 2:45 a.m. While it is known that one suspect is down in Watertown, it is still not clear whether or not the shooting at MIT and the firefight in Watertown are related to the Boston Marathon bombings.
2:54 a.m. Watertown resident and eyewitness Andrew Kitzenberg tells MSNBC that authorities have deployed a bomb-squad robot near where the shootout occurred.
4:16 a.m. Two law enforcement sources tell NBC News that the suspect pictured in the black hat is dead, and the suspect in the white hat is at-large and considered armed and dangerous. The officials say the shootings at MIT and in Watertown are directly related to the Marathon bombings.
4:19 a.m. Officials in Watertown ask all residents to shelter-in-place.
4:35 a.m. Watertown police officers continue to search the neighborhood on foot and in patrol cars.
5:01 a.m. Officials tell NBC News that the two suspects in the bombing and the slaying of the MIT officer appear to have international ties and may have military experience. The deceased suspect was strapped with an improvised explosive device, according to the officials.
5:20 a.m.-6:30 a.m. Local universities and colleges including Harvard, Boston University, Emerson College, Boston College and MIT cancel classes and tell students to remain in place. Boston Public Schools suspend all activities.
5:45 a.m. Public transit (MBTA) service is canceled throughout the city.
6:30 a.m. Amtrak service into and out of Boston South Station is delayed by police activity. Train service between Boston and Providence, R.I., is temporarily suspended.
6:40 a.m. NBC's Ron Allen reports a convoy of about 15 military Humvees and a few civilian buses heading into Watertown.
7 a.m. More than 400,000 people shelter-in-place in the neighborhoods of Cambridge, Newton, Waltham, Brighton, Watertown and Allston-Brighton. Authorities say the suspects are brothers.
8 a.m. Massachusetts Gov. Deval Patrick urges all residents in Boston and the surrounding area to remain indoors as authorities engage in a "massive manhunt." The shelter-in-place instruction is extended across the city.
About 10 a.m. – The deceased MIT police officer is identified as Sean Collier, 26, of Somerville. A former civilian employee of the Somerville Police Department, he had served at MIT since January 2012, according to a release from the Middlesex district attorney.
12:30 p.m. – Authorities request that residents remain in their homes, saying that about 60 percent of the area they want to search in Watertown had been covered without an apprehension.
6 p.m. -- Authorities lift the order for people stay in their homes and reopen Boston transit. Gov. Deval Patrick says people must remain vigilant because "there is still a very, very dangerous individual at large." Col. Timothy Alben of Massachusetts State Police says suspect has not been apprehended but vows that he will be.
7 p.m. -- A barrage of gunfire is heard in a Watertown neighborhood.
7:35 p.m. -- A person is reported to be in a boat in the backyard of Watertown home. Authorities say resident saw blood leading to the boat and thermal imaging then showed a person inside it.
8:05 p.m. -- Police move in on the boat and believe the suspect is hiding there.
8:45 p.m. -- Suspect Dzhokhar Tsarnaev, 19, is captured alive, police say.
Around 9 p.m. -- Boston Police Department tweets: "CAPTURED!!! The hunt is over. The search is done. The terror is over. And justice has won. Suspect in custody."
— NBC News' Pete Williams, Ron Allen, Tom Winter, Michael Isikoff, Erin McClam, John Bailey and Richard Esposito contributed to this report.