The official, who asked not to be named because the investigation is ongoing, said Tuesday that Officer Jeff Stenroos told investigators that he had accidentally fired a weapon.
The official says Stenroos used a gun that was not his service weapon.
Detectives continue to investigate Stenroos and are skeptical about his differing accounts of what happened. The officer's bulletproof vest was apparently struck by a bullet, though it's not clear if he was wearing it at the time.
Stenroos has not responded to requests for comment since the Jan. 19 incident.