United States police are hunting for a suspect in the deadly Boston Marathon bombing in a small town near the city.
On Monday, two bombs were set off near the marathon's finish line, killing three people and wounding at least 170.
A huge police operation began on Thursday after an officer was shot dead at the Massachusetts Institute of Technology (MIT) campus in Cambridge.
Police said the officer was responding to a report of a disturbance in the area of the MIT campus about 10.30pm when he was shot and suffered "multiple wounds".
Police said one bombing suspect was killed during a car-chase and shoot-out in nearby Watertown, 10km from Boston. Another suspect, shown in images from the marathon wearing a white baseball cap, is believed to be on the run in Watertown, the BBC reports.
Police said these men are suspected of killing the officer. They then stole a car at gunpoint and drove away with the car's owner still on board. They later released the driver unharmed.
The men threw bombs and traded gunfire with police before being stopped at Watertown. An officer was critically wounded in the chase.
Images released by the US Federal Bureau of Investigation earlier on Thursday show two men walking together through the crowd at the marathon. The man in the white hat was seen planting the bag that eventually exploded, it said.
Meanwhile, investigators are combing through thousands of pieces of evidence, from cell phone pictures submitted by spectators to shards of shrapnel pulled from the legs of victims, Reuters reports.
US President Barack Obama on Thursday told thousands of mourners at an interfaith prayer service in Boston that that everyone had been touched by the attack on their beloved city.
At the service at the Cathedral of the Holy Cross, Mr Obama paid tribute to the three people who lost their lives in the and said bombs could not beat Boston, or America.