'She was screaming': Calif. Highway Patrol officer caught on video pummeling woman
CHP officer Daniel Andrew is filmed pummeling Marlene Pinnock on July 1, 2014. [KNBC-TV]

An unidentified California Highway Patrol officer is under investigation after getting caught on video beating a woman about the head a dozen times with no apparent indication that he was in danger.

KNBC-TV reported on Thursday that the incident was filmed Tuesday afternoon in Los Angeles by motorist David Díaz as he was making his way onto Interestate 10.

Díaz's 90-second clip begins with the officer catching up to the woman on foot. She appears to walk away from him, before the officer takes her to the ground. After a brief struggle, the officer begins pummeling her with punches to the head. The woman can be seen covering up and not fighting back.

"He basically got on top of her, and it's basically a UFC ground-and-pound move," Díaz told KTLA-TV. "It's basically full force, punching her in the head."

After the flurry, a second man enters the frame and helps the officer restrain the woman. KNBC reported that the other man was a plainclothes officer at the scene.

"She was screaming," another witness, Daina Tate, told KNBC. "Two men were on her. They had her arms up. She couldn't defend herself. She looked totally helpless and terrified. It was shocking, and you can't really call the cops on the cops."

A third witness said to KNBC that the unidentified woman was not wearing any shoes at the time of the encounter and was carrying "lots of bags."

The CHP did not identify the circumstances leading up to the encounter, but released a statement saying it was under investigation.

"As a matter of policy, every time there is a use of force by our officers, there is a review conducted to determine whether the use of force was appropriate," the statement read. "That will be done in this case, however, since there is an ongoing investigation, it would be premature to comment on this specific video segment without reviewing the entire incident."

Watch KNBC's report, as aired on Thursday, below.