-- From Times staff reporter Christine Clarridge:

Seattle police concede it's "very embarrassing" that a semi-automatic rifle was left on the back of a patrol car.

The AR-15 was left on the trunk lid of the patrol car that was parked outside of the Roosevelt Hotel on Pine Street around 9 p.m. Monday. A man snapped a photo of the rifle and flagged down bicycle patrol officers, Seattle Police Department spokesman Sgt. Sean Whitcomb said.

The same man also notified The Stranger and sent the alternative weekly the photo, police said. According to The Stranger, the car was unattended when the photo was snapped.

Another witnesses reportedly followed the cruiser while the rifle was on the trunk. However, Whitcomb would not say whether the cruiser was driven while the AR-15 was on the trunk.

Whitcomb said the incident "is very embarrassing and shouldn't have happened. It's unacceptable."

He said the captain of the West Precinct will launch an investigation into the circumstances that led to the AR-15 being left on the patrol car. Whitcomb would not speculate as to whether the officer would face discipline.

He said leaving a weapon unattended is a violation of the department's policy, but not a criminal violation.

Whitcomb says such rifles are assigned only to officers who have additional training. They're usually kept in the trunk or between the driver and passenger seats.