A Santa Rosa man with a loaded, stolen gun in his car was arrested Thursday night during a traffic stop in Petaluma, police said.

A Petaluma police officer pulled over a car driven by Christopher Rosen near East Washington and Ellis streets at 10:06 p.m. after noticing an expired vehicle registration tag, police said.

The officer was later notified that Rosen was under a domestic violence restraining order and was possibly armed, police said. Rosen denied having any firearms.

Police towed Rosen’s vehicle because the registration had been expired for more than six months. During a search of the vehicle, they discovered a loaded .45 caliber handgun under the driver’s seat.

Rosen was arrested on suspicion of possessing a loaded stolen firearm, possessing stolen property and violating a domestic violence restraining order, police said.