WATERTOWN, Massachusetts — Authorities said Friday that one of two suspects in the Boston Marathon bombing is dead and a massive manhunt is underway for another. Residents of the Boston suburb of Watertown were advised to keep their doors locked and not let anyone in. The Middlesex district attorney said in a statement that the two men were suspected of killing an MIT police officer at the college late Thursday, then stealing a car at gunpoint and later releasing its driver unharmed. Hours earlier, police had released photos of the marathon bombing suspects and asked for the public’s help in finding them. Authorities said the suspects threw explosives from the car as police followed it into Watertown. The suspects and police exchanged gunfire and one of the suspects was critically injured and later died. Police described the second suspect, still at large, as “heavily armed and dangerous.” “We believe this to be a terrorist. We believe this to be a man who has come here to kill people. We need to get him in custody,” a police official on the scene told a press conference. FBI officials said earlier Friday that they had taken into custody a man who had “similar features” to one of the suspects sought for the tragic bombings that occurred during the Boston Marathon earlier in the week. Police, National Guard and FBI cordoned off a Boston-area town and were searching for at least one other suspect Friday morning. Law enforcement officials who had converged on Watertown, some 10 miles (16 kilometers) west of Boston, were telling reporters to turn off their cell phones and leave the area, saying, ”If you want to live, turn off your cell phone.” A transit police officer was wounded in Watertown and evacuated to a local hospital, according to CNN. Authorities at the scene early Friday were heard 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.” In Watertown, witnesses reported hearing multiple gunshots and explosions at about 1 a.m. Friday. 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.” Boston cab driver Imran Sais said he was standing on a street corner at a police barricade across from a diner when he heard an explosion. “I heard a loud boom and then a rapid succession of pop, pop, pop,” he said. “It sounded like automatic weapons. And then I heard the second explosion.” He said he could smell something burning and advanced to check it out but area residents at their windows yelled at him, “Hey, it’s gunfire! Don’t go that way!” Earlier Thursday night a campus police officer was shot and killed at MIT. The MIT officer was responding to report of a disturbance when he was shot multiple times, according to a statement from the Middlesex District Attorney’s office and Cambridge police. It said there were no other victims. MIT said on its website that police were sweeping the campus in Cambridge and urged people to remain indoors until further notice. People were urged to stay away from the Stata Center, a mixed use building with faculty offices, classrooms and a common area. The shooting came little more than three days after the twin bombings on the Boston Marathon that killed three people, wounded more than 180 others and led to an increase in security across the city. The shooting took place about 10:30 p.m. local time Thursday outside an MIT building, according to Procopio. The officer, who was not immediately identified, was taken to a hospital where he was pronounced dead from his wounds. About 11,000 people attend the prestigious university.