A man who was arrested during a traffic stop early Wednesday died later in the day in the custody of Milwaukee police.
An investigation has been launched, and three officers have been placed on administrative duty in line with department policy, police said in a news release.
The man, 20, whom police did not identify, was pulled over just before 2 a.m. Wednesday for a traffic violation. He was arrested after officers learned he had a warrant.
Police said a search of his car also turned up illegal narcotics.
The man was booked at Milwaukee Police Department District 5. Authorities said jail staff members "conducted multiple cell checks" and that at around 6 p.m. a staffer noticed that the man was in need of medical attention.
"Officers provided life-saving measures until they were relieved by the Milwaukee Fire Department. Ultimately, the individual was pronounced deceased," police said.
Chief Jeffrey Norman said to his knowledge the man was not involved in any physical altercations during his arrest and had not been injured before he was booked.
Norman said the case remains under investigation.