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.

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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.

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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.

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