(Reuters) - One suspect in the Boston Marathon bombing was killed during a shootout with police and a massive manhunt was underway for the second suspect in the Boston suburb of Watertown, officials said on Friday.

Police were searching for the bombing suspect who was photographed wearing a white hat just before the explosions that killed three people and wounded 176. The blasts triggered security scares across the United States and evoked memories of the September 11, 2001 attacks.

"We believe this to be a terrorist," said Boston Police Commissioner Ed Davis of the bombing suspect still at large. "We believe this to be a man who has come here to kill people. We need to get him in custody."

The massive police operation was under way in Watertown after the U.S. Federal Bureau of Investigation (FBI) on Thursday released pictures and video of two suspects in the Boston Marathon bombing, 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.

Massachusetts State Police warned people in Watertown not to open their doors while police conducting a door-to-door, street-by-street search.

HOW THE NIGHT BEGAN

About five hours after the FBI released the pictures of the bombing suspects, a police officer was shot and killed on the campus of Massachusetts Institute of Technology, the Middlesex County District Attorney said in a statement.

A short time later, police received reports of a carjacking by two men who kept their victim inside the car for about half an hour, the statement said.

Police pursued that car to Watertown, where explosives were thrown from the car at police and gunfire was exchanged, the statement said.

"During the exchange of the gunfire, we believe that one of the suspects was struck and ultimately taken into custody. A second suspect was able to flee from that car and there is an active search going on at this point in time," Colonel Timothy Alben, superintendent of the Massachusetts State Police, told a news conference.

"What we are looking for right now is a suspect consistent with suspect No. 2, the white-capped individual who was involved in Monday's bombing of the Boston Marathon," Alben said.

(Additional reporting by Alex Dobuzinskis in Los Angeles and David Bailey in Minnesota; Writing by Daniel Trotta; Editing by Mary Milliken and Lisa Shumaker)