US police are hunting a suspect in Monday's deadly Boston Marathon bombing in a small town near the city.
A huge police operation started after an officer was shot dead on a university campus.
One suspect was killed during a car-chase and shoot-out in Watertown, six miles (10km) from Boston.
Another suspect, shown in images from the Boston Marathon wearing a white baseball cap, was on the run in Watertown.
Police said in a statement the two men were suspected of killing a police officer on campus at the Massachusetts Institute of Technology (MIT) late on Thursday.
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 FBI has released several images of the suspects
The men threw bombs and traded gunfire with police before being stopped at Watertown.
Witnesses heard several explosions and a gun-battle lasting 10 minutes.
Video footage emerged of a suspect lying on the floor, and later being led into a police car after being stripped of his clothes.
Police later said one suspect had been hit by bullets during the chase and later died.
It was unclear whether the arrested man was the suspect who had died.
Police also confirmed that an officer had been critically wounded in the chase.
Police later said they were looking for "suspect number two", the "white-capped individual".
The FBI has released several images of two men they were hunting in relation to Monday's marathon bombing.
One was wearing a white cap, the other had a black cap.
Boston police Commissioner Ed Davis said he believed the man being hunted in Watertown was a "terrorist".
"We believe this to be a man who came here to kill people," he said.
Three people were killed and more than 170 wounded when the two bombs exploded near the finish line of the Boston Marathon.
The blasts killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, 23, a postgraduate student from China.
A huge police operation started after an officer was shot dead on a university campus.
One suspect was killed during a car-chase and shoot-out in Watertown, six miles (10km) from Boston.
Another suspect, shown in images from the Boston Marathon wearing a white baseball cap, was on the run in Watertown.
Police said in a statement the two men were suspected of killing a police officer on campus at the Massachusetts Institute of Technology (MIT) late on Thursday.
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 FBI has released several images of the suspects
The men threw bombs and traded gunfire with police before being stopped at Watertown.
Witnesses heard several explosions and a gun-battle lasting 10 minutes.
Video footage emerged of a suspect lying on the floor, and later being led into a police car after being stripped of his clothes.
Police later said one suspect had been hit by bullets during the chase and later died.
It was unclear whether the arrested man was the suspect who had died.
Police also confirmed that an officer had been critically wounded in the chase.
Police later said they were looking for "suspect number two", the "white-capped individual".
The FBI has released several images of two men they were hunting in relation to Monday's marathon bombing.
One was wearing a white cap, the other had a black cap.
Boston police Commissioner Ed Davis said he believed the man being hunted in Watertown was a "terrorist".
"We believe this to be a man who came here to kill people," he said.
Three people were killed and more than 170 wounded when the two bombs exploded near the finish line of the Boston Marathon.
The blasts killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, 23, a postgraduate student from China.
