Gunfire and explosions erupted in streets close to the prestigious Massachusetts Institute of Technology on Friday after a police officer was shot dead there, witnesses said.
Local media said one person may have been shot in the incidents as heavily armed police launched a major manhunt following the killing on the MIT campus near the US city of Boston.
The explosions and gunfire were heard in the Watertown district north of the university. "There were blasts, it could have been grenades," local resident Adam Brown told AFP.
The NBC-WJAR channel later showed film of a man lying on the ground in a street in the town and surrounded by police.
Two hours earlier, a gunman shot dead an MIT campus policeman.
MIT is in Cambridge, Massachusetts, just across the Charles River from Boston, where three people were killed and 180 injured in a double bomb attack on the city's marathon on Monday. Authorities made no immediate link between the two incidents.