Tucson Police in Turmoil After Death of Latino Man in Custody

Chief Magnus’s own offer to resign seemed to catch Ms. Romero, who was standing by his side, by surprise. She said she would examine the details of what happened before taking action.

The department’s handling of the issue is now coming under intense scrutiny. Authorities did not disclose details about Mr. Lopez’s death until Tuesday, when Ms. Romero canceled a City Council meeting after watching the video.

Before the release of the video, Chief Magnus had publicly described the Tucson police force as one of the more progressive departments in the country. It had previously banned chokeholds and required officers to participate in cultural awareness and crisis intervention training.

Chief Magnus said that officers were responding to a call regarding “disorderly conduct” by Mr. Lopez, who was unclothed and seemed to be acting erratically when the officers arrived at the scene. At one point, an officer told Mr. Lopez he would be shocked with a stun gun if he failed to cooperate.

In the news conference, Chief Magnus said he had asked the F.B.I. to review the episode, which has been under internal investigation in the department. He said the officers involved had not met the standards established in training for what he described as a mental health crisis involving “excited delirium.”

For years, many departments have trained officers that people held face down, in what is known as “prone restraint,” are more likely to die suddenly of positional asphyxia because they have difficulty expanding their chest to bring in air.

This is particularly true if they are showing signs of mental distress or intoxication with stimulant drugs, a condition sometimes referred to as excited delirium. Guidelines for such circumstances usually call for officers to move people onto their side or sit them up as soon as possible.