Neighbours reported hearing several gunshots and explosions in Watertown on Thursday, a neighbouring suburb to the Massachusetts Institute of Technology (MIT) where a campus police officer was killed.
At least one suspect was in police custody from the Watertown incident, the ABC TV station in Boston reported. Reuters was not immediately able to verify the reports.
It was not clear if the police activity in Watertown was related to the shooting at MIT, or if either was connected to the bombing of the Boston Marathon, which killed three people and wounded 176.
Investigators released pictures and video of two suspects in the Boston Marathon bombing on Thursday, enlisting the public's help in identifying two men wearing backpacks and baseball caps in the crowd minutes before bombs exploded near the finish line.
Police officers with weapons drawn, and people wearing Federal Bureau of Investigation (FBI) and Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) jackets could be seen in the live video and long afterward police continued to call the scene active.
Boston police called it an "active incident" ongoing in Watertown and urged residents to remain inside.
Three loud explosions
Rebecca Carbone (30) stood on the street wearing pyjamas and a sweatshirt and had stepped out when she heard sirens.
"We heard a loud blast and we didn't know what it was," Carbone said. "It sounded like a car backfiring."
John Grimes (69), a retired letter carrier, said that he heard three loud explosions, "and you don't hear explosions at night a lot".
MIT early on Friday said MIT police had determined that the suspect in the shooting was no longer on campus and it was now safe to resume normal activities.
David Procopio, a spokesperson for the Massachusetts state police, said late on Thursday that the motive for the shooting was not known and the investigation would be led by the Middlesex County District Attorney's Office after the death of the officer.
Procopio said he could not "say that with 100% certainty at this time" that the shooting at the MIT campus was not related to the Boston Marathon bombing. He could not be reached for comment after the Watertown incident.
FBI Boston Special Agent Greg Comcowich said the office was referring questions about the MIT shooting to Cambridge and MIT police. "I can tell you that we are aware of it however."
MIT police representatives could not be reached immediately for comment. – Reuters