FBI spokeswoman Laura Eimiller says a civil rights inquiry was initiated Thursday after the video, allegedly of an arrest on Aug. 11, came to the bureau's attention.
The footage apparently shows two officers holding down William Cardenas, 24, on a sidewalk as one punches him several times in the face before they handcuff him. One of the officers appears to have his knee on the man's throat. The struggling suspect yells repeatedly, "I can't breathe!"
Police say they have begun their own criminal and administrative investigations into the officers' use of force during the arrest. A police spokesman identifies the officers as Alexander Schlegel and Patrick Farrell; both have been reassigned to administrative work.
"There's no denying that the video is disturbing," said Police Chief William Bratton, at a news conference. "But as to whether the actions of the officers were appropriate in light of what they were experiencing and the totality of the circumstances is what the investigation will determine."
Authorities learned of the tape in late August after the defense turned it over to prosecutors. The district attorney's office is reviewing whether to continue with the Cardenas case, which is scheduled for trial Nov. 20, said spokeswoman Jane Robison.
It was unclear who shot the footage, which is not an official police video or from a surveillance camera. The video was also posted on Youtube.com.
Police spokesman Lt. Paul Vernon said Cardenas, a known gang member who had been wanted on a felony warrant for receiving stolen property, ran after police encountered him. After catching up to him, the officers knocked him to the ground and arrested him.
Cardenas, who was held without bail, faces two charges of resisting arrest.
Andre Birotte, the LAPD's independent watchdog, says his office was informed of the arrest in October and is monitoring the investigations.
Ingumenson said he welcomed the investigations, contending the officers acted well within their rights and department policy. In particular, police appeared to use what are known as "distraction strikes," a tactic for subduing suspects, he said.
"This would have never happened if the suspect had surrendered as he is lawfully obligated to do," Ingumenson said.
Cardenas' attorney, B. Kwaku Duren, accused the officers of violating his client's civil rights and claimed department investigators were stalling.
"I think the LAPD is being caught covering up an obvious excessive use of force," he said.