Boston citizens are asked to "remain vigilant" as police search for the second of two Boston Marathon bombing suspects who were first identified on the 18th. After being identified on video by the FBI, a pair of brothers — 19-year-old Dzhokhar A. Tsarnaev and 26-year-old Tamerlan Tsarnaev — attempted to flee Boston on the night of the 18th, killing police officer Sean Collier in the process. Contrary to initial reports, they were not responsible for robbing a nearby convenience store. The pair hijacked a van but were trapped in a shootout with police, during which at least one more explosive device was detonated and Tamerlan Tsarnaev was killed.

His brother Dzhokhar escaped, and the police continue what is by now an hours-long manhunt, methodically searching Watertown, MA and the surrounding areas. Most public events have been canceled, and public transportation was shut down for most of a day while citizens were asked to remain in their homes. As of a press conference at 6pm ET, that informal lockdown was lifted, and the subway was re-opened, but police warn that Tsarnaev is still at large.

While police have not yet apprehended Dzhokhar Tsarnaev, information about his past and family have come to light, sparking widespread speculation about the suspects and their motives. Both were immigrants with a long past in Boston. A photo essay from 2010 indicates that Tamerlan was a competitive boxer, though it has now been made private. Dzhokhar has been identified as a student at the University of Massachusetts Dartmouth, where his father has said he was studying medicine. Both Tsarnaev brothers came to the US in the early 2000s, and an uncle describes them as both Muslims and ethnic Chechens. In an emotional statement to the press, however, he denied that either Chechen politics or Islam had any role in the attacks.

Friends of Dzhokhar Tsarnaev have expressed disbelief that he would be capable of the bombing, but his parents have gone one step further, accusing unknown parties of staging a setup. While the Tsarnaev brothers' uncle called the two "losers" and urged Dzhokhar to turn himself in, his father and aunt argued that the photographs released yesterday were staged, while Dzhokhar and Tamerlan's mother said she was "100 percent sure" the two were not responsible for either the bombing or subsequent killing of Collier. In an interview, she claimed that Dzhokhar had been "controlled" or tracked by the FBI for five years, after the agency apparently pegged him as a person of interest.

What we know about the Boston manhunt so far:

Boston Police have just confirmed that they are engaged in an ongoing search for a suspect consistent with the FBI's description of "Suspect #2" released earlier Thursday in connection with the Boston Marathon bombing on Monday which killed three people and injured nearly 200. The police spokesperson also confirms that they have killed another man, identified as "Suspect #1" by the FBI.

The events began at 10:20PM, when gunshots were reported at MIT in Cambridge, where one police officer was found with multiple gunshot wounds and later pronounced dead at Massachusetts General Hospital. Shortly after, at around 12:30AM, there were reports of a carjacking in which two armed suspects reportedly stole a Mercedes SUV. According to local news reports, the suspects kidnapped the victim, holding him for a half hour before releasing him.

The police reportedly followed the suspects from Cambridge to nearby Watertown, about ten minutes away. Recounting the night's events to The New York Times, Watertown resident Andrew Kitzenburg describes two young men shooting at "dozens of police officers officers from behind a black Mercedes SUV." At some point, the two shooters, says Kitzenberg, detonated some kind of explosive device. Police have since confirmed that at least one explosive device was used, and that one suspect was killed. They have also confirmed that a transit officer was shot in the firefight.

The story has been tracked nearly in real-time across Twitter, Facebook, and other social networks as people around the world tune in and try to make sense of the news. Local police scanner channels have been a focus, with concurrent listener numbers totaling more than 80,000. Images and updates from local reporters as well as national journalists have been flooding in at an unprecedented pace, creating a deep pool of sometimes-confusing or contradictory information.

The Boston Globe reports that police were "screaming about improvised explosive devices," and a bomb-detecting robot was deployed in the area to look for undetonated explosives.

Police have a set up a twenty block perimeter area around Watertown and stress that a manhunt is currently underway. They have advised local citizens to stay in their homes and not answer their doors. Police have released another surveillance photo of the man who is wanted for questioning, and commissioner Edward Davis had this to say: "We believe this to be a terrorist. We believe this to be a man who has come here to kill people. We need to get him in custody."

Update 5:23AM ET: A press conference was held at the hospital where "Suspect #2" was brought after the altercation with police. Doctors say it was an "adult male patient brought in under police guard," adding that the man had "multiple injuries, we believe a combination of a blast injury and multiple gunshot wounds."

Update 6:22AM ET: Police continue to search for the remaining suspect and residents in a 20-mile radius around the town of Watertown have been advised to stay in their houses and not try to use public transport as all MBTA service is currently suspended.

Update 6:57AM ET: Both NBC News and the AP claim to have named the surviving marathon bomber suspect. However, this name has not been confirmed by officials.

Update 7:28 AM ET: American singer-songwriter Juliana Hatfield, from Boston, woke up to a note on her front door from the City of Cambridge.

Update 7:55AM ET: The Boston Regional Intelligence Center has identified 19 year-old Dzhokhar A. Tsarnaev as one of the suspects in the Boston Marathon bombings who is still at large.

Update 8:15AM ET: Both bomb suspects have been named as brothers Dzhokhar A. Tsarnaev (19) and Tamerlan Tsarnaev (26). Dzhokhar A. Tsarnaev is still at large. Boston citizens have been informed to stay indoors as the manhunt is still ongoing. All taxi service in the City of Boston has been suspended.

Update 8:29AM ET: Boston Police Department is still hunting for 19 year-old marathon bomb suspect Dzhokhar A. Tsarnaev.

Update 9:24AM ET: The FBI has released a second photo of the suspect, seen below.

Update 9:46AM ET: The Cambridge Chronicle and Boston Globe report that the officer killed in last night's shootout was 26-year-old Sean Collier of Somerville, MA.

Update 10:21AM ET: Several sources, including the Boston Globe, are reporting that Connecticut Police have put out a bulletin for a gray Honda CRV. NBC reported earlier that the police are looking for a second living suspect.

Update 10:25AM ET: The Boston police have now confirmed the search for a possible suspect car:

Update 10:45AM ET: Police have found the vehicle in question unoccupied and are processing it for evidence.

Update 11:15AM ET: Amtrak train service has been "suspended indefinitely" between Boston and Providence, RI. Television news has shown police gathering around an area in Watertown with guns drawn, though there's no clear picture of what's going on.

Update 11:40AM ET: Chechen President Ramzan Kadyrov has released a statement on his popular Instagram account, responding to allegations that the suspects were spurred to the bombing by their roots in Chechnya. The statement is given in Russian, but as translated, it expresses condolences to Boston, then denies that there is any connection between Chechen culture and the Tsarnaev brothers' motivation. "They grew up in the United States, their attitudes and beliefs were formed there," he writes, saying that people must "look for the roots of their evil" in America.

Update 12:59PM ET: Amtrak service between New York and Boston has been suspended as well.

Update 1:56PM ET: CBS, which correctly reported earlier that the police were looking for a suspect vehicle, also said they're seeking another: a 1999 green Honda Civic. This was confirmed soon after by the Boston Police Twitter account. However, about an hour later, police allegedly canceled the search, saying that "We have that car. We are not looking for it."

Update 4:26PM ET: Earlier in the day, reports circulated of a possible third suspect, but the highly reliable Pete Williams of NBC now reports that police are "focused solely on Dzhokhar Tsarnaev."

Update 5:12PM ET: Russia Today has spoken to the mother of the two suspects, who says she is "100 percent sure" they were set up and could not have committed the bombings. More strangely, she also claims that one of her sons was "controlled by the FBI" for five years, that they monitored his web traffic, and that the FBI was afraid of him because "they think he is a leader. He talks about Islam a lot." While the FBI claims were not echoed by the suspects' father, he has also maintained that the two had been set up, and an aunt said earlier today that photographic evidence of them at the Boston Marathon finish line was "staged." None have provided an explanation for the subsequent shootout, which also involved improvised explosive devices.

Update 6:15PM ET: In a press conference, police lifted the self-imposed lockdown on Boston, urging citizens to "remain vigilant" but saying that they were free to leave their homes. Boston's subway system will also be resuming operation. Unfortunately, the suspect remains at large. "We do not have an apprehension of the suspect this afternoon," said Col. Timothy Alben, "but we will have one." Police say that though they do not know precisely where Tsarnaev is at this point, they believe he has not left Massachusetts. They also said that contrary to initial reports, the suspects did not rob a convenience store while attempting to leave Boston; instead, they were only in the area at the time of an unrelated robbery.

Update 7:06PM ET: A rapid burst of gunfire in the Watertown neighborhood was just reported. The Boston Globe is reporting a source as saying that a suspect is "pinned down" somewhere in the area. A body has apparently been found "on a boat in a backyard."

Update 7:11PM ET: We're now hearing from CNN that a suspect — likely Tsarnaev — is cornered by police. Gas is being fired to subdue him. Watertown residents are once again being asked to stay inside, just minutes after the lockdown was lifted.

Update 7:43PM ET: Police have detected "movement" inside the boat, located in the backyard of a home; the boat is surrounded.

Update 8:16PM ET: The Washington Post is now reporting confirmation from authorities that the man pinned down in the boat is 19-year-old Dzhokhar Tsarnaev.

Update 8:41PM ET: Tsarnaev has been captured alive. There are reports of cheering at the scene; the suspect is said to be "badly injured" after having lit a fire inside the boat where he was holed up, possibly using a 40-gallon tank that had been on the boat with him.

Update 9:01PM ET: Boston police commissioner Ed Davis gives mayor Tom Menino the good news, in this picture:

Update: 9:15PM ET: The FBI has also confirmed that Dzhokar Tsarnaev is in custody.

This post will be updated as news develops.