CHICAGO (CBS) — A man died Thursday afternoon while he was in the custody of the Waukegan police.

Waukegan police said detectives responded to a residence in the 600 block of Martin Luther King Jr. Avenue around 3 p.m. to serve a search warrant for narcotics. They observed a man in his 30s with an active arrest warrant pull up to the residence. He fled when detectives approached him.

READ MORE: Mayor Lightfoot Delays General Iron Permit To Move To South Side Over EPA Concerns

As he was running away, the man began to eat an unknown white substance. He fell and was arrested by police, continuing to chew the unknown substance.

READ MORE: Chicago Night Clubs Gear Up For Looser COVID-19 Restrictions As State Prepares To Enter Bridge Phase

The subject refused to spit out the substance and eventually became physically distressed. The Waukegan Fire Department was called, and the man was transported to an area hospital where he was pronounced dead.

The Lake County Illinois Coroner’s Office expects an autopsy to be scheduled for Friday. The office will release the subject’s identity once his family has been properly notified.

MORE NEWS: Saint Joseph Catholic School Principal On Administrative Duty After Former Teacher Charged With Child Sex Crimes

The case is being investigated by Illinois State Police, at the request of Waukegan Police Chief Wayne Walles.