A police officer has been gunned and a second officer critically injured after a night of chaos in Boston.
One officer, whose identity has not been released, was shot after attending a disturbance at Massachusetts Institute of Technology campus.
A second officer identified as Richard H. Donahue, 33, is in a critical condition after attending a shoot-out between police and two Boston bombing suspects in the suburbs of the city.
The drama unfolded hours after the FBI released images of two men named as suspects in the Boston blast which killed three and injured 176 on Monday.
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Fatally injured: A police officer has been gunned down after attending a disturbance at Massachusetts Institute of Technology campus
Combing the area: Police search an area of the Massachusetts Institute of Technology after the officer was gunned down
Hunt: Policemen patrol on the MIT campus in Boston, the scene where a police officer was shot dead
The Middlesex district attorney said the two men are suspected of killing the MIT police officer on campus in Cambridge late last night, then stealing a car at gunpoint.
The officer was responding to a report of a disturbance in the area of the MIT campus at around 22:30 when he was killed.
He was taken to hospital where he was pronounced dead - no-one else was hurt.
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It is unclear what the disturbance at the prestigious campus was, but it is currently on lockdown.
Police are patrolling the grounds of the university's Stata Cente and the university later issued a statement to say the campus is now safe but urged students to 'remain vigilant'.
Clues: Massachusetts State Police, Cambridge Police and MIT Police search at the scene of the shooting
Vigilant: Police search neighborhoods yard by yard after a police chase and shootout with two heavily armed men
Medical help: An ambulances drives into the prestigious campus following reports of the shooting
A few hours later a second officer was injured in a shoot-out - police said the officer is currently in a critical condition.
The officer was hit after a dramatic exchange of gunfire in the Watertown area, some six miles (10km) from Boston not long after the incident at the university.
It is unclear if the Watertown shooting is related to the earlier incident.
Dozens of police officers, FBI agents, national guard, K-9 units and SWAT teams rushed to the area after midnight following a reported car chase involving the two suspects in a black 2013 Mercedes.
Armed and dangerous: Police are hunting for the terrorist dubbed 'Suspect 2' (left) after he went on the run following a firefight in the Watertown suburb of Boston. The other, named 'Suspect 1' (right) died in hospital after the gun battle
The shoot-out came hours after these images were released in the hunt for two 'armed and extremely dangerous' men
Police say the two suspects - who are believed to be responsible for the Boston bombings - discharged several explosives at police from the vehicle during the pursuit.
According to eyewitness reports, two men believed to be in their early twenties engaged in a furious gun fight with dozens of police on a backstreet of Watertown.
The exchange left one of the Boston bombing suspects dead - dubbed 'Suspect One' by the FBI and the second - the 'White Cap Suspect Two' is still on the loose.
Police said Richard H. Donahue, a transit officer was injured in the gunfight.
Authorities said he was in surgery at Mount Auburn Hospital.
Boston Police Chief Ed Davis tweeted: ‘One suspect dead. One at large. Armed and dangerous. White hat suspect at large.’
The drama unfolded hours after the FBI released images of two men named as suspects in the deadly Boston marathon attacks and the Boston Globe has said the two men involved in this evenings shoot-out are those same men.
The images show them walking in single file toward the finishing line of the race around 13 minutes before the twin detonations.
Hours after the initial photos were made public, investigators released close-up shots this morning of the two suspects that more clearly show the men's faces.
University officials said MIT is heartbroken by the news that an MIT Police officer was shot and killed in the line of duty on Thursday night on campus'
Massachusetts State Police spokesperson Dave Procopio said that they believe multiple possible explosive devices were used against police tonight during this incident at Watertown
Suspect Number One was wearing a black baseball hat, a dark hooded jacket and khaki pants. Suspect Number Two was wearing a white basketball cap on backwards and a dark hooded sweatshirt.
The FBI made no mention of the men's height, weight or age range and would not discuss the men's ethnicity.
'It would be inappropriate to comment on the ethnicity of the men because it could lead people down the wrong path potentially,' said FBI agent Greg Comcowich, a spokesman for the Boston FBI office
The information on the first suspect was developed within a day or so before its release, DesLauriers said.
Agent Daniel Curtin said the FBI did not issue the photos earlier because authorities wanted to be meticulous: 'It's important to get it right.'
And according to FBI sources who have spoken to CNN, still unreleased footage reveals that the suspects stayed at the scene to watch the devastation the two blasts created.
'When the bombs blow up, when most people are running away and victims were lying on the ground, the two suspects walk away pretty casually,' said the official to CNN, who has seen the unreleased video.
'They acted differently than everyone else,' he added.
Following last night's killing MIT has cancelled it classes today.
A letter from Israel Ruiz, the school's executive vice president and treasurer, and school Chancellor Eric Grimson.
'MIT suffered a tragedy last night: an MIT Police officer was shot and killed on our campus in the line of duty,' says the letter, addressed to the MIT community. 'While the circumstances around the officer's death remain the subject of an active investigation, what is certain is that the officer gave his life to defend the peace of our campus. His sacrifice will never be forgotten by the Institute. We are thinking now of his family, and our hearts are heavy.