This is a screenshot of video obtained by Channel 2 Action News of the alleged incident. (Photo: Channel 2 Action News)

But things didn’t end there.

“The inmate was safely subdued initially, but immediately began self-harming behavior as soon as the deputies left her cell,” Volkodav said. “Deputies entered the cell again to prevent the inmate from harming herself. It was during this exchange that Aaron Masters is seen on video striking the inmate in the head three times with a closed fist.”

According to a warrant, Clark suffered a black eye and swollen cheek. The sheriff’s office and the Gwinnett County District Attorney’s Office charged Masters with battery.

Masters is out of jail on bond. It’s unclear if Clark, who was in the Gwinnett County jail on a simple battery charge, was charged in the assault against Masters and the other deputies.

The incident occurred exactly a week before court filings revealed that a federal grand jury is conducting a criminal investigation into the agency’s “rapid response team” — and argued it was “highly unlikely that any criminal activity will be uncovered.”

Masters, who had been with the sheriff’s office for three years, was part of the RRT squad but had no disciplinary history, Volkodav said. Sheriff Butch Conway admonished Masters’ behavior.

“With great power comes great responsibility,” he said in a statement. “Our deputies work hard every day to provide a safe community for citizens, inmates and staff alike. There’s no room for a quick temper in this job and this behavior will not be tolerated.”

The controversial RRT squad takes part in what the sheriff’s office describes as “high-risk tactical operations” inside the county jail. Federal authorities did not identify a particular RRT member or act as the subject of the investigation, which was broad in nature and focused on the activities of the squad.

The filing was made as part of an ongoing federal lawsuit filed in 2013 over the RRT's use of "restraint chairs," devices designed to hold unruly inmates' arms, legs, chest and head in place. The civil lawsuit — filed in U.S. District Court in Atlanta, initially on behalf of a dozen or so former Gwinnett inmates — alleges that the chairs amount to excessive force and were inappropriately used as punishment.

— Staff writer Tyler Estep contributed to this article.

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