Updated 02/27/14 – 12:38 p.m.

OAK LAWN, Ill. (CBS) — Police said they can’t yet explain why an Oak Lawn man killed his parents, his 5-year-old nephew, and then himself on Wednesday, and acknowledged they might never solve the gruesome mystery.

Oak Lawn Police Chief of Detectives Michael Kaufmann said investigators believe 36-year-old John Conta Jr. stole a shotgun from a locker where his father kept his hunting weapons early Wednesday, then systematically shot and killed his father, John Sr.; his mother, Janice; and his nephew, Matthew Meier.

Conta Jr. then set fire to the house he shared with his parents in the 9800 block of South 51st Avenue, sat in a car in the garage, and killed himself with the same shotgun just as an assistant fire chief walked in while responding to a 911 call.

The night before, the Contas had been celebrating little Matthew’s 5th birthday with a sleepover.

“The 5-year-old was staying overnight, as the grandparents adored him. It was his birthday the night before this happened,” Kaufmann said.

According to Kaufmann Conta Jr. has no history of mental illness, and his motive remains a mystery. He said everything seemed normal in the days leading up to the shootings.

Relatives have told police Conta Jr.’s personality seemed to change after he spent a year living in Russia as a potato farmer.

“That may have been a contributing factor, but there’s nothing to lead us to believe something happened in the last two days that would have set him off to do a rampage like this,” he said. “It’s possible we may never know.”

Detectives are still trying to piece together the last hours of Conta Jr.’s life in an effort to determine a motive.

The South Suburban Major Crimes Task Force was assisting in the investigation.