The FBI has said that it would open an investigation after 10 California sheriff's deputies were filmed apparently beating a suspect who had attempted to flee on horseback.
Footage captured by the KNBC news helicopter shows the deputies repeatedly punching and kicking Francis Pusok, 30, who appeared to have surrendered after he was shot by a Taser stun gun.
The 10 San Bernardino County deputies have been suspended with pay.
Sheriff John McMahon told reporters at a press conference that he was "disturbed and troubled" by what he saw in the video.
"It appears to be excessive to me based on what I saw in the video," he added.
Pusok was treated at a local hospital for abrasions and bruises, then jailed on several criminal charges, including felony evasion and possession of stolen property, McMahon confirmed.
He had initially fled in a car after officers turned up at his home with a search warrant as part of an identity-theft investigation, Reuters reported.
He eventually abandoned his vehicle and stole a horse before deputies in off-road vehicles and helicopters closed in on him as he made his way through rugged terrain near the town.
The video shows Pusok riding the horse until he falls off, briefly scrambling to his feet, then throwing himself to the ground, on his stomach with his arms out in front of him, as two deputies run up with Tasers drawn.
The deputies then proceed to punch and stomp on Pusok, landing several blows to the head.
The incident lasted for two minutes before the deputies arrest him.
The sheriff confirmed that the Taser was ineffective because Pusok was wearing loose-fitting clothing.
Two deputies were treated for minor injuries and released, the sheriff said.
This is the latest example of police brutality in the US to garner global attention.
Last week, South Carolina police officer Michael Slager was charged with murder after footage showed him apparently firing eight bullets at 50-year-old Walter Scott as he was running away.
Slager had previously stated that he shot Scott after he tried to take the officer's Taser.