A police officer has been shot dead and armed police have converged on a neighbourhood near Boston amid reports of explosive devices being used against them.
The dramatic developments came hours after the FBI released photos and video of two suspects in the Boston Marathon bombings, urging the public to help identify the men but warning people not to approach them.
TV footage showed armed officers surrounding a suspect lying on the ground in Watertown outside Boston.
Dozens of officers and National Guard members were in the area and television outlets reported that gunfire and explosions had been heard.
State police spokesman David Procopio said: “The incident in Watertown did involve what we believe to be explosive devices possibly, potentially, being used against the police officers.”
Witnesses reported hearing multiple gunshots and explosions at about 1am (6am Irish time). Dozens of police officers and FBI agents were in the neighbourhood and a helicopter circled overhead.
Authorities were calling for somebody to get on the ground and put their hands up and a loud thud was heard after someone shouted “fire in the hole”.
Reporters were told to move away from the scene. A police officer told a reporter: “If you want to live, turn off your cell phone.”
Earlier, a campus police officer was shot and killed at Massachusetts Institute of Technology in Cambridge, near Boston, and the authorities were searching for the person responsible.
The dead officer had been responding to report of a disturbance when he was shot several times, according to a statement from the Middlesex district attorney’s office and Cambridge police. It said there were no other victims.
MIT said right after the 10.30pm shooting that police were sweeping the campus in Cambridge and urged people to remain indoors. They urged people to stay away from the Stata Building.
Hours later, MIT, which has about 11,000 students, said the campus was clear but the gunman was still on the loose.
Police are not yet officially linking the developments to the Boston Marathon bombings but they occurred hours after the images of the bomb suspects were released.
The bomb suspects’ images came from surveillance cameras near the explosion sites shortly before the blasts near the marathon finish line.
FBI Agent Richard DesLauriers said: “We consider them to be armed and extremely dangerous.”
Monday’s blasts killed three and injured more than 180.
The images were released hours after President Barack Obama attended an interfaith service in Boston to remember the victims, including an eight-year-old boy.
The FBI’s website crashed within moments of the images being released.
They show two young-looking men wearing baseball caps, jackets and carrying backpacks along the race route and weaving through the crowd.
“Each piece moves us toward justice,” Mr DesLauriers said of the latest information to emerge.
He said one suspect is believed to have planted the explosive devices near the finishing line of the world-famous marathon.
The authorities had warned people not to expect the case to be cracked quickly as they looked through a vast response to their plea for photos and video from the blast scene.
The images came out hours after Mr Obama promised a grieving city those responsible would be hunted down.
At the interfaith service honouring the victims, the president called the perpetrators of the attack “these small, stunted individuals who would destroy instead of build”.
The blasts killed eight-year-old Martin Richard, 29-year-old Krystle Campbell and Lu Lingzi, a graduate student from China. Seven victims remained in critical condition.
The bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings, investigators said. They suspect the devices were then hidden in duffel bags and left on the ground.
They exploded within 15 seconds of each other near the finishing line at a time when thousands of runners were pouring in.