Severe damage remained after a fatal house fire in the 5000 block of N. Oak Park Ave. View Full Caption DNAinfo/Alex Nitkin

NORWOOD PARK — A 75-year-old man found in a burning Norwood Park home last week was stabbed to death, and his death is being investigated as a murder, authorities said.

At 5:55 a.m. Friday, firefighters responded to a call of heavy smoke coming from a single-family home in the 5000 block of North Oak Park Avenue, according to Chicago Fire Department Cmdr. Frank Velez.

The fire was put out within 20 minutes of their arrival, Velez said.

While searching the home, responders came across an "elderly" man lying unconscious inside, Velez said. The man was taken to Resurrection Medical Center, where he was pronounced dead.

Officers later said they were treating the death as a murder and were investigating it.

The Cook County medical examiner identified the man as 75-year-old Loren Flensborg. An autopsy released Tuesday showed he was stabbed to death.

The man was unmarried and had lived in the house alone for at least 30 years, neighbors said.

"He mostly kept to himself, but he was always friendly and willing to help other people with whatever projects they were working on," said Al Janda, who lived across the street from Flensborg.

Flensborg was a landlord who managed several properties in Lakeview, Janda said, adding that his neighbor was an "avid Cubs fan."

Family members said Flensborg may have known whoever killed him, as there were no signs that someone forced their way into the home.

"It looked like he let in someone he knew," a sister told the Tribune.

Flensborg's girlfriend described Flensborg, who grew up in the home, as "a giant teddy bear," the Tribune reported.

Linda Ayapam, who lives on the same block, left her home to walk her dogs around 5:45 a.m. Friday when she saw "thick black smoke" pouring from the house, she said.

"This is a really quiet area, so when something like this happens, it moves you," Ayapam said. "It's a sad day for our block."

The cause of the fire remains under investigation, Velez said.