Heather Locklear was arrested on suspicion of driving under the influence Saturday evening in Santa Barbara County, Calif.

"Around 4:30 p.m., a female witness saw Heather Locklear driving erratically while leaving a parking lot in Montecito," California Highway Patrol spokesman Tom Marshall tells PEOPLE. "The witness said Locklear was revving her engine loudly, and backed over a pair of sunglasses several times."

Soon after, the witness – who apparently didn't recognize the actress – spotted Locklear, 47, stopped on a nearby highway, exiting her vehicle.

"The witness called 911 because she was concerned for Locklear's safety," Marshall said. "When a CHP officer arrived, Locklear was found seated inside her vehicle, which was partially blocking the roadway."

Because Locklear appeared "disoriented and impaired," she was taken to a local CHP station and drug and alcohol tests were administered, Marshall said. The actress was cooperative.

"Alcohol was ruled out as a factor, but based on the officer's observations, we believe Locklear was under the influence of prescription medication," Marshall added.

Locklear was arrested on suspicion of DUI and being under the influence of a controlled substance. She booked at a local jail around 7 p.m., and released hours later.

The actress's lawyer, Blair Berk, wasn't immediately available for comment. Locklear's publicist had no comment.

Just two months ago, Locklear left an Arizona treatment facility after battling anxiety and depression.