HESPERIA, Calif. (AP) — A news video shows California sheriff's deputies tracking down a man fleeing on horseback and then punching and kicking him dozens of times when he's on the ground.

The San Bernardino County Sheriff's office ordered an investigation into the beating Thursday.

"The video surrounding this arrest is disturbing," San Bernardino County Sheriff-Coroner John McMahon said.

A sheriff's statement said Francis Pusok, 30, fled from deputies early in the afternoon as they tried to serve a search warrant in an identity-theft investigation. He eventually abandoned a car in the Hesperia area and stole a horse, but was tracked down by deputies.

KNBC-TV (http://bit.ly/1H8UC3D ) helicopter footage shows the man dressed in bright red clothing falling from the horse as a deputy runs up and uses a stun gun on him. The sheriff's department statement said the stun gun was ineffective.

The man falls face-down with his arms and legs outstretched and puts his hands behind his back. The video shows two deputies appearing to come up and kick him in the head and crotch. Other deputies arrive moments later.

KNBC-TV said up to 13 deputies eventually surrounded the man, and some of them kicked, hit and punched him dozens of times over a two-minute period.

"It doesn't look good. It looks like his hands are behind his back even when they're doing the blows," said Ken Cooper, a use of force expert. "The justification for using force is to gain compliance from the suspect, and the suspect seems to be complying. So what this looks like is those blows are not justified, they're not necessary and they're not professional."

Cooper said the officers should be disciplined, retrained to deal with stress especially, and the video should be used for department-wide training moving forward. He said the officers let their emotions and the adrenaline overtake their professionalism. But training helps "inoculate" officers from responding improperly during high-stress situations.

"When chasing a fleeing suspect, in high stress, you have to control that. It's your obligation as a professional. You can lose it sometimes," Cooper said.

Pusok's attorney said to him the video showed "thugs beating up my client."

"These questions about what was he doing, what were they doing?" attorney Jim Terrell told KCAL-TV.

The beating of Pusok, who is white, came as recent violent episodes by law officers dealing with suspects have provoked outrage after being captured on video, including the shooting death of an unarmed black man as he ran from a police officer last weekend in North Charleston, South Carolina.

Pusok's girlfriend of 13 years Jolene Bindner said he has had several run-ins with the law but is a great father.

"I'm not going to stand here and say that he's perfect, because who is?" she told the TV station.

"I couldn't believe it," Bindner said after seeing the video. "The first thing I said was 'they cannot do that.'"

The American Civil Liberties Union released a statement saying it is "deeply troubled by the video images" and applauding McMahon's call for an investigation.

Pusok was taken to a local hospital with unknown injuries, according to the Sheriff's Department.

Two deputies suffered dehydration, a third was kicked by the horse and all were taken to a hospital for treatment, the statement said.

Copyright 2015 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.