The Pleasanton Police Department on Tuesday asked for help identifying a man who impersonated a police officer, faked a traffic stop and then sexually assaulted a female driver earlier this month.

What sounded like a police siren chirping and a flash of light led the female driver to pull over.

Investigators said she was driving on Valley Avenue at Bernal at 3 a.m. on Sunday, August 5th.

"The male suspect left his car, walked up to her car and ordered her out of her car," explained Pleasanton Police Sgt. Maria Munayer. "Before he did that, he displayed a badge in his hand."

Police said the suspect handcuffed the woman and put her in his unmarked sedan. He drove a short distance and parked on an unpaved road.

"In the back seat of the car, he sexually assaulted her and brandished a pistol. After it was over, he left in his car," said Munayer.

The woman managed to walk back to her car.

Police explained that all law enforcement officers in California are required to have a solid red light on while conducting a traffic stop. Police said individuals are not required by law to stop if you do not see that light on the vehicle.

One woman said it's always unsettling to be pulled over.

"How can you make the assumption that it's not the police?" asked Annie Lin of Pleasanton.

Munayer advised drivers to slow down, put on their vehicle's blinkers and drive to a well-lit area and call 911 if you have any doubts.

"Sexual assaults are rarity here in town. We are a bit surprised," said Munayer.

Police described the suspect as a white man in his late 20's or early 30's with dark hair, five feet ten to six feet tall with a muscular build.

They said he wore a dark shirt and a dark police-style hat with a patent leather brim that had no police insignia.

Pleasanton Police asked anyone with information to call them at 925-931-5100.