The hunt for the two brothers accused of carrying out the Boston Marathon bombings evolved rapidly through Thursday night and the early hours of Friday. Authorities ordered a lockdown of Boston and several surrounding suburbs after engaging in an overnight battle with the suspects, who police say were armed with guns and explosives. The two men, who authorities said have international connections, allegedly shot a campus police officer to death at the Massachusetts Institute of Technology, authorities said. The pair then fled in a hijacked SUV only to engage in a firefight with police in the suburb of Watertown. The gun battle left one of the brothers dead. The other escaped and was still on the run.
Here's how the violent events have transpired so far, 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 second wearing a black cap. The two men were each carrying backpacks in the footage. The FBI said 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 their 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 the area is cordoned off by responders.
Around 10:30 p.m. - An MIT police officer is found shot in his vehicle. He is taken to Massachusetts General Hospital and pronounced deceased. Shortly after the shooting, police receive a report of an armed carjacking in a separate section of Cambridge. The carjacking victim is held at gunpoint by two males for a half hour before being released unhurt at a gas station on Memorial Drive in Cambridge, according to a statement by the Middlesex District Attorney.
Some time after this - it's unclear exactly when - police engage in a pursuit into Watertown following the carjacked vehicle. Explosive devices are reportedly thrown from the SUV, according to the district attorney's statement, and an MBTA police officer is seriously injured. One of the suspects is critically injured, and later pronounced deceased, 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. Around this time, the suspects tried to use a debit card stolen from the man whose car they jacked to withdraw money from three ATMs, a person with direct knowledge of the incident told NBC's Pete Williams. The first attempt was unsuccessful, but they withdrew $800 on the second attempt. At the third ATM, at 11:20 p.m., a withdrawal was denied for exceeding the man's daily limit.
11:41 p.m. - 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 Massachusetts 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 were reports of violence. Several ambulances were also on the scene.
1:31 a.m. - MIT advises people on campus to remain indoors. "Police have NOT determined that the campus is safe."
1:51 a.m. - Tom Winter reports that the National Guard and FBI are on 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 that lands in the space between themselves and the police, 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 MIT cop slaying appear to have international ties and may have military experience. The deceased suspect had an improvised explosive device strapped to him, officials say.
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 Scools suspended all activities.
5:45 a.m. - All 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 head 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 two suspects are brothers.
About 7:30 a.m. - The two suspects are identified as brothers. Dzhokhar Tsarnaev, 19, was identified as the suspect in the white hat and was still at-large. He was born in Kyrgyzstan. His brother, Tamerlan Tzarnaev, 26, was born in Russia, authorities said. He was the deceased suspect.
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.
NBC News' Pete Williams, Ron Allen, Tom Winter, Michael Isikoff, Erin McClam, John Bailey and Richard Esposito contributed to this report.
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