The men were brothers and died from gunshot wounds, the coroner said. The house belonged to their elderly mother, who died earlier this week.
Highland Park fire and police officials responded around 3 a.m. to the 1700 block of Park Ave. West for a report of flames shooting from the roof of a home.
Lt. Chris Covelli with the Lake County Sheriff's office told ABC7 the deaths have been classified as a homicide investigation.
"They located 2 adult males outside of the home and they both had injuries that were not consistent with fire related injuries," Covelli said. "They're looking at a number of things, they're looking into potential video surveillance in the area, identified who the house was owned by, who may have been inside the home, who had access to the home."
The Lake County Coroner's office has identified the victims as brothers, 63-year-old Marc Austwick of Lake Forest and 60-year-old John Austwick of Mundelein. Both died from multiple gunshot wounds.
Police said a woman in her 80s lived in the home, but she passed away five days ago. Neighbors said the elderly homeowner's son stayed with her at times.
Phyllis Austwick, the mother of Marc and John Austwick, lived in the home and died May 31 at Evanston Hospital from a medical issue, the coroner confirmed.
The home sits on an acre of property and is set back from the road.
"It looks like the fire was extending feet up in the sky, and it was just kept going and going and going," Lisa Spathis, who lives nearby, said.
She said she woke up to her dog barking. Spathis looked outside and saw the house ablaze.
"It was crazy," Spathis said. "It wasn't a small fire. It was igniting. And saw all the trucks out and everyone running around and it was crazy."
Firefighters located the two deceased men outside of the home, but no other victims were found on the property.
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Hours later into Wednesday afternoon, investigators continued to search the property for clues to what happened.
"I thought it was suspicious. I got an alert from the city that there was a fire on this block, and I had a feeling it was this house," resident Metra Richter said.
The investigation brought out a swarm of first responders from several neighboring fire departments, including the Lake County Major Crimes Task Force as well as state and federal authorities.
"They went door to door to the homes around here showing a picture of a young male and asked if he looked familiar," resident Julie Smith said.
Neighbors said the home has been up for sale on and off for years.
"There was a dumpster that they brought back there a couple days ago," resident Judy Aberman said. "This house has been for sale forever and the land next to it, so it's quite disturbing to see something like that in our neighborhood."
Neighbors said the family lived in the neighborhood for decades but kept to themselves. ABC7 was also told the homeowner's son stayed with her at times, but police have not released the identities of the victims.
"I mean, it's shocking," Spathis said. "And it's sad very sad and scary."
Park Ave. West was closed between Ridge and Sunnyside during the investigation.
Lake county investigators said they did not recover any weapon on the property.
The investigations into the double-homicide and the cause of the fire continue, authorities said.