The Lake County Sheriff's Department says Julius Boros, 83, had been shot several times inside in unincorporated Dyer.
Residents of the quiet development where Boros lived are stunned.
"I just can't believe it. I really can't. I'm so sorry for the family," said neighbor Hilda Kuiken.
Neighbors say Boros was found by one of his sons. Neighbors also say the relatives regularly checked on Boros, whose wife died a year and a half ago.
"The vice president of our association was there and he called me and said that Julius had been assaulted and murdered, and we were just -- we couldn't believe it," said Barb Hoffstedt, neighborhood association president.
Hoffstedt sent a letter to residents warning them to keep their patio doors locked.
"I feel very secure in this neighborhood, and I'm pretty sure most of our homeowners still feel safe and secure," said Hoffstedt. "We don't want anybody to panic. We're treating it or thinking about it as an isolated incident."
Family members describe Boros as being a happy-go-lucky guy who did not deserve to be killed.