Officials said John Petrik suffered a heart attack in a police car following this week's fire, during which officers had to drag Petrik from his home after he refused to leave behind his 13-year-old German shepherd mix, the Chicago Tribune said Thursday.
The Cook County medical examiner's office said Petrik died early Wednesday morning following the fire while his dog Regis is alive but in intensive care at a veterinary hospital.
The dog was rescued by firefighters after Petrik was taken from the home.
Veterinarian Dr. Cesar Agustin told the Chicago Sun-Times Petrik, who had no living relatives, was a very loyal owner to Regis.
"He was a shy man -- very quiet, very private. But he was very attached to that dog. He loved that dog," Agustin said.
A preliminary investigation into the blaze indicates it may have begun due to a electrical short in a space heater.