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The San Bernardino County, California, Sheriff's Office opened two investigations Thursday only hours after an NBC Los Angeles helicopter recorded deputies using a stun gun on a man on a stolen horse and then beating him repeatedly.

In the video, a sheriff's helicopter can be seen landing next to the man, who falls off the horse and is stunned by one of the deputies. Two deputies begin punching him in the head and kneeing him in the groin. Then, three others arrive and join in the pummeling, which lasts about two minutes.

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The sheriff's office identified the man in a statement as Francis Jared Pusok, 30, of Apple Valley. It said Pusok was being treated at a hospital for undisclosed injuries.

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The sheriff's office said deputies were serving a search warrant in connection with an identity theft investigation about 12:12 p.m. (3:12 p.m. ET) when Pusok fled in a vehicle. At some point, he stole a horse from a group of people in San Bernardino National Park and rode off through dirt trails into steep, rugged terrain, the statement said.

About two hours after the chase started, three deputies landed in a sheriff's helicopter, spooking the horse, which threw Pusok off, according to the statement. "A Taser was deployed but was ineffective due to his loose clothing," it said. "A use of force occurred during the arrest."

Cynthia Bachman, a spokeswoman for the sheriff's department, said that two deputies were being treated for dehydration and that a third was injured. That deputy was kicked by the horse, which was also injured during the pursuit, the sheriff's statement said.

"The video surrounding this incident is disturbing and I have ordered an internal investigation be conducted immediately," Sheriff John McMahon said. He said a special detail was also conducting a criminal investigation.

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Pursuit on horseback ends w/deputies stunning man with Taser, beating him, @TonyNBCLA reports. http://t.co/7hyrvFFHf7 pic.twitter.com/B1MpQ0B6zq — NBC Los Angeles (@NBCLA) April 9, 2015