The California Highway Patrol said Friday that it is investigating a video of one of its officers holding down and repeatedly punching a woman on the side of a Los Angeles freeway. The video, captured on another driver's cell phone, shows the woman lying on the ground while the officer straddles her and hits her multiple times in the head.

David Diaz, the passing driver who recorded the assault, witnessed the event Tuesday and offered the captured footage to media outlets. He told the Associated Press in a phone interview Friday that the officer "agitated the situation more than helped it."

CHP Assistant Chief Chris O'Quinn said at a news conference that the woman was walking on Interstate 10, west of downtown Los Angeles, endangering herself and other drivers. When the officer answered a report of a pedestrian on the freeway, he found her walking down an off-ramp. Upon seeing him, the woman walked back onto the freeway, wandering in and out of lanes of traffic.

The video shows that once the highway patrol officer caught up to the woman, he pinned her down and subdued her with many blows to the face and head until a man in plain clothes, later identified as an off-duty law enforcement officer, intervenes to help get her into handcuffs.

The officer was trying to restrain her and the incident report listed no injuries to the woman, who is undergoing psychiatric evaluation, said O'Quinn.

The video "only shows a small part of what transpired," said O'Quinn. He would not comment on what led the officer to act in this manner, but did note that CHP is trained in the heightened dangers of being on the freeway compared to a citizen "who is not accustomed to the speed and conditions."

The names of the woman and the officer involved have not been released, although the officer is on administrative leave pending the investigation.