A police officer passing by saw the fire at the tavern. Before it was under control three alarms were called, with 12 departments responding.
Firefighters tried to save George Markopoulos, 67, part owner of the bar, but he did not survive.
Rumors about foul play circulated Thursday but the West Suburban Major Crimes Task Force is calling it a death investigation.
"We don't know how the fire started at 3:30 in the morning, and the person who was in the building was laying on the floor. So we don't know how that happened. So that is why it's suspicious," said Officer Michael Marrazzo, Schiller Park Police.
Police say there was no sign of trauma on Markopoulos' body.
"I worried about him all the time. Because he never came home, he always stayed here and he was always worried about his bar," said Helen Rofus, Markopoulos' ex-fiancee.
Rofus says they were reconciling and had seen each other Wednesday night. She heard the news of Markopoulos' death Thursday.
"He was a caring guy. He would give the world his life...and evidently he did give his life," said Rofus.
"He would get out of bed to help someone. He would pull money out of his pocket that he needed for himself to help someone," said Ryan Rofus, friend.
Friends say Markopoulos recently offered shelter to a homeless woman.
"I don't know what happened. It is crazy. He is a really good guy," said Elias Nakos, friend.
An autopsy is scheduled for Friday.