Last Updated Apr 10, 2015 1:47 PM EDT
Aerial footage from Los Angeles station KNBC shows Francis Pusok, 30, on horseback, fleeing from San Bernardino County deputies in California. It also appears to show those deputies using a Taser and beating Pusok on the ground.
Authorities have yet to say whether the man seen in the video was armed, but after seeing the footage, the city's sheriff admitted that what he saw was "disturbing." A criminal investigation has been launched, reports CBS News correspondent Carter Evans.
Deputies are seen firing Tasers, bringing Pusok down.
"Suspect being tased. Suspect being tased," deputies can be heard saying.
While on the ground, two deputies appear to unleash a fury of kicks, knees and punches to Pusok's head and body with his face down and hands behind his back.
More deputies arrive and the beating continues. Over nearly two minutes, Pusok is struck more than 50 times.
"I couldn't believe it. The first thing I said is, 'They can't do that,'" Pusok's girlfriend Jolene Binder said. "I see him put his hands out behind his back and I just see officers swinging their legs. Kicking him punching him."
Deputies said they were trying to serve a search warrant on Pusok for identity theft, but he ran away from them, stole a horse and led them on a two hour chase near the Deep Creek area of Apple Valley.
"There was an attempt to take him into custody and a use of force occurred," spokesperson Jodi Miller said. "That use of force is now under investigation."
Pusok has had several run ins with the law dating back to 2002, including attempted robbery, driving on a sidewalk at an unsafe speed and obstructing or resisting an officer in 2014.
"I'm not going to stand here and say that he's perfect, but who is," Binder said.
"They're asking the family member what was he doing there. No, the focus has got to be on these cops," attorney Jim Terrell said. "These are bad cops."
Pusok was taken to the hospital with unknown injuries but Binder, mother of his three children, said she did not know his condition or even what hospital he was taken to.
She said they are considering a lawsuit.
Deputies claim three of their own were injured during the pursuit; one kicked by the horse Pusok allegedly stole and two suffering dehydration. Identities of the deputies involved in the arrest have not been released.