MIT Police Officer Sean Collier, 26, of Somerville, was killed Thursday.

MIT Police Officer Sean Collier, 26, of Somerville, was killed Thursday.

A suspect in the Monday bombing at the Boston Marathon finish line may be on the loose but hunted by police in Watertown as of 2:45 a.m. Friday, with a police scanner report going out for officers to look out for a man with black curly hair wearing a charcoal gray hoodie with green-on-orange lettering, possibly armed with an assault rifle and explosives. He should be considered “extremely dangerous,” police said.

As of 3:40 a.m., police had sealed a 20-block perimeter in Watertown and planned a street-by-street, house-by-house and backyard search as dawn broke. Residents were being asked to stay indoors as teams of two canvassed.

“This is a terrorist. We believe this to be a man who’s come to kill people,” Boston Police Commissioner Ed Davis said at a 4:20 a.m. press conference.

As the sky lightened at 5:43 a.m. Friday, the MBTA said that at the request of the police all service is shut down until further notice. There was to be no transport into or out of Watertown, as well. Residents of Cambridge, as well as Watertown, Waltham, Newton, Belmont and Allston/Brighton were reminded to stay indoors by police, and it was suggested that businesses not open.

MIT campus police officer Sean Collier, 26, was shot to death in his cruiser at 10:20 p.m. by two men late Thursday was only the start of the night’s madness. Shortly afterward the men carjacked a Mercedes E350 on Third Street, keeping the owner with them for about a half-hour until dropping him unharmed at the Mobil gas station at 816 Memorial Drive, according to the Middlesex District Attorney’s Office, police and media reports. The stolen car was reported headed toward Harvard Square – and then, at 12:50 a.m. Friday, came reports of shots fired, “explosives and some kind of grenades” leading to a “loud explosion” in Watertown. As police pursued the suspects into Watertown, the D.A.’s office said,

Explosive devices were reportedly thrown from car by the suspects. The suspects and police also exchanged gunfire in the area of Dexter and Laurel streets. During this pursuit, an MBTA Police officer was seriously injured and transported to the hospital.

The MBTA officer was identified as Richard H. Donahue Jr., 33.

The Boston Globe reported at around 2:30 a.m. that there was a connection between the incidents in Cambridge and Watertown and Monday’s bombing, which killed three people and injured 176. The suspect considered free at 3 a.m. was believed to be the bombing suspect seen in photos wearing a white cap, although an hour later a Massachusetts State Police spokesman was saying only that in terms of a link to Monday’s violence, “We’re investigating that and the FBI is investigating that.” The other suspect was being reported as dead.

Incredibly enough, when police released suspects’ names over the scanner, they were given as Mike Mulugeta and Sunil Tripathi – a Pennsylvanian who was once a student at Brown University in Providence, R.I., who had been missing since March 15. (NBC said the bombing suspects were from overseas and had military training, which would eliminate Tripathi.)

At the time, Tripathi was on break from school. Described as “the most shy, considerate, gentle young man” by his sister in The Brown Daily Herald, Tripathi was considered depressed, and at first it was thought he might have committed suicide. By late March, he had been spotted in surveillance video and there was hope he was alive, and the FBI had become involved in the search. (A description of the suspect being hunted Friday was of a white man with a dark complexion, or someone of Middle Eastern descent, but Tripathi is of Indian descent.)

At 6:45 a.m., though, The Associated Press said there was another suspect: Dzhokhar A. Tsarnaev, 19 – a 2011 graduate of Cambridge Rindge & Latin School and wrestler who got a $2,500 city scholarship.

This image released by the FBI shows two suspects in the Monday bombing of the Boston Marathon. The man at left, wearing a white cap, was being hunted by police in Watertown just before 3 a.m. Friday.

This image released by the FBI shows two suspects in the Monday bombing of the Boston Marathon. The man at left, wearing a white cap, was being hunted by police in Watertown just before 3 a.m. Friday.

Off to Watertown

People tracking the progress of the chase picked up large amounts of gunfire and possibly explosives at Lowell Street in Watertown. (The Mercedes was ultimately found at Lincoln and Spruce streets or Lincoln and Walnut streets.)

The shootout in Watertown that involved heavy explosives resulted in one suspect being injured – and taken to Beth Israel Deaconess, where he died – and another on the run. With one still being searched out, plainclothes officers were warned to stay out of the backyard search so they weren’t taken shot at by mistake. The warning would be repeated.

By 1:24 a.m. Friday, a second person was in custody, but this was just one of dozens of false starts police had over the several-hour ordeal. Officers weren’t sure if the person was an actual suspect, but by 1:39 a.m., after the man had been stripped and put in a cruiser, it was confirmed there was still a suspect on the loose. Many hours later, the search was still ongoing, with action reported at the Newton border and at Watertown’s Arsenal Mall area. The suspect appeared to have moved south.

Officers were warned at 4:42 a.m. that the first suspect had explosives on him that went off when police moved in, and doctors at Beth Israel later confirmed the suspect died from a “combination of blast injury and gun wounds” to the trunk. He died after about 15 minutes. The second suspect might have explosives as well, an unidentified officer said.

Over the course of the night, at least three officers were injured, but the gathered forces were estimated in the thousands. The law enforcement in Watertown included SWAT, FBI, National Guard, Homeland Security and ATF, according to various sources. Figures ultimately were estimated Friday at between 6,000 and 10,000. “I have never before seen this show of force,” said Josh Brogadir, a NECN anchor who was on the scene.

The first shooting

Massachusetts State Police and Cambridge Police are investigating the fatal shooting of the MIT officer, which took place at MIT’s Stata Center, also known as Building 32. The Tech reported more:

A postdoctoral student working in Building 76 called the MIT Police at about 10:25 p.m. to report loud sounds, possibly gunshots. The injured officer was found at 10:31 p.m. and was transported to Massachusetts General Hospital.

According to  responding to Middlesex Acting District Attorney Michael Pelgro and Cambridge Police Commissioner Robert Haas, the MIT officer was found with multiple gunshot wounds after responding to a report of a disturbance at Vassar and Main streets.

According to WBZ-TV:

The brothers wanted in the Boston Marathon bombings ambushed Collier, shot him five or six times and once in the head. Collier never drew his weapon. His gun was found in his holster.  Five 9 mm shells were found at the scene.

“Our thoughts and prayers are with the officer’s family and our brothers and sisters at the MIT Police,” Cambridge police said via Twitter.

Collier was an MIT police officer since January 2012 and before that was a civilian employee with the Somerville Police Department. The Boston Globe published a lengthy profile of him.

Collier was an MIT police officer since January 2012 and prior to that he was a civilian employee with the Somerville Police Department. MIT Police Chief DiFava described Collier as a dedicated officer who was extremely well liked by his colleagues and the MIT community.

Authorities launched an immediate investigation into the shooting. It was joined by the Federal Bureau of Investigation, according to The Harvard Crimson, and people on the scene reported K9 units and helicopters being used to search. As the violence and response moved on to Watertown, officers kept searching the campus area, announcing just before 2 a.m. that the suspect was no longer there, MIT News reported. “It is now safe to resume normal activities,” the office said. Several hours later, Friday classes were canceled. Harvard followed, citing public safety concerns.

The scene of an MIT police officer shooting early Friday involved city police, State Police, K9 units and helicopters. (Photo: Jess Bidgood)

The scene of an MIT police officer shooting early Friday involved city police, State Police, K9 units and helicopters. (Photo: Jess Bidgood)