The California Highway Patrol officer caught on video straddling a woman on the ground and punching her in the head on a Los Angeles freeway was trying to restrain her, CHP Assistant Chief Chris O'Quinn said Friday at a news conference.
The woman was walking on Interstate 10 on Tuesday; O'Quinn said she was endangering herself and people in traffic.
The not-yet-identified officer is on administrative leave while the CHP investigates.
Here's the clip of the incident recorded by driver David Diaz from inside his passing car. (Content warning: Rough language accompanies the disturbing images):
O'Quinn says the woman, who would not give her name, was not injured and is undergoing psychiatric evaluation.
The CHP provided KABC-TV with a written statement, noting that the officer ordered the woman to stop, but she ignored him. Fearing she would be hurt, the CHP says the officer tried placing her under arrest.
"A physical altercation ensued as the pedestrian continued to resist arrest, at which point a plain clothes, off duty officer assisted in applying the handcuffs," the CHP's statement also said.
Diaz said he saw the officer chasing the woman around a pickup truck on the onramp before he started recording.
The Associated Press contributed to this story.