The Maricopa County Sheriff's Office has identified the man as 44-year-old Ernest M. Atencio.
Phoenix police brought Atencio in for booking on suspicion of assault at about 3 a.m. on Friday. During the booking process, Maricopa County sheriff's officials said Atencio was abusive and combative, forcing police and sheriff's deputies to use "defensive efforts" in restraining him.
In a statement issued by the sheriff's office, MCSO Deputy Director Jack MacIntyre was quoted as saying the officers took Atencio to a "safe cell" in hopes of getting him under control. While in the cell, Atencio was under observation by medical personnel, MacIntyre said. About 15 minutes later, medical staff checked on Atencio and had to start CPR and other revival efforts, McIntyre said.
Atencio was taken to St. Joseph's Hospital in Phoenix.
McIntyre said an investigation is ongoing.