The Mesa Police Department in Arizona on Tuesday voluntarily released disturbing surveillance video footage showing a group of Mesa police officers brutally beating an unarmed man while responding to a call at an apartment complex in May.

The 20-minute video, taken from a surveillance camera, shows several police officers kneeing and punching a black man, now identified as Robert Johnson. At one point, they slam his head into an elevator door.

The incident took place on May 23 when officers responded to a 911 call from a local apartment complex. They arrived to find Erick Reyes and Robert Johnson. Officers ordered Reyes to sit and began speaking with him, but their interaction with Johnson, who was unarmed and talking on a cellphone, quickly escalated.

Mesa Police Chief Ramon Batista told reporters that he was releasing the video to the public because he wanted to be transparent about the incident and provide additional context that would be left out if the video were anonymously posted online. Batista said he was not aware of the incident until the surveillance video was sent to the police department by a civilian.

“I don’t feel that our officers were at their best,” Batista told reporters Tuesday when releasing the video. “I don’t feel this situation needed to go the way that it went.”

Four of the officers involved in the incident have been placed on administrative leave while the department conducts an investigation, and Batista said that the department would change some of its use-of-force policies after the incident. More videos related to the incident are expected to be released later this week.

“This in no way represents the whole work that is done every day,” Batista said. “They’re human beings and certainly at first glance, this looks like a mistake.”

But critics of the department’s behavior say several high-profile incidents involving the Mesa Police Department in the past few years complicate the efforts to cast this as a one-off event.