A teenager was shot and killed by a police officer in the Bronx on Thursday afternoon after running into his home as officers pursued him, the authorities said.

Paul J. Browne, the New York Police Department’s chief spokesman, said there was “no evidence that he was armed” when the officer, a member of a narcotics unit, shot him once in the upper left chest. It was unclear what had prompted the chase, Mr. Browne said.

The teenager, Ramarley Graham, 18, was pronounced dead at Montefiore Medical Center.

During the pursuit, Mr. Graham turned onto East 229th Street and entered his family’s house, Mr. Browne said, adding that a team of officers followed him inside. The team was a plainclothes unit, though the group does wear raid jackets with police insignia on the front and back and shoulder patches on each arm. The officer had struggled with Mr. Graham near the entrance to a bathroom, Mr. Browne said, before shooting him. A small amount of marijuana was found in the toilet, Mr. Browne said.

It was unclear whether the gun, a 9-millimeter semiautomatic, was fired during the struggle or if the men had been separated when the shooting occurred, Mr. Browne said. Another officer and a sergeant at the scene were being interviewed by the police on Thursday night.