Detectives are investigating the death of an off-duty officer found Monday in his car on the side of the Long Island Expressway as an apparent suicide, police said.

Nassau County police identified the officer only as a 39-year-old man from Suffolk County.

He was discovered in his car on the shoulder of the eastbound LIE near Exit 40 in Jericho at about 3:40 p.m. by a passerby investigating what was thought to be a disabled car, police said. The passerby then called 911.

The road was closed for investigation until about 6 p.m.

From celebrating America's 250th birthday to a new ride at Adventureland, NewsdayTV's Elisa DiStefano and Newsday lifestyle editor Meghan Giannotta have your inside look at Newsday's summer FunBook. Credit: Newsday/Howard Schnapp, Kendall Rodriguez, Drew Singh; Anthony Florio, Randee Daddona, Morgan Campbell, Debbie Egan-Chin

Get ready for sun and fun with NewsdayTV's summer FunBook special! From celebrating America's 250th birthday to a new ride at Adventureland, NewsdayTV's Elisa DiStefano and Newsday lifestyle editor Meghan Giannotta have your inside look at Newsday's summer FunBook.

From celebrating America's 250th birthday to a new ride at Adventureland, NewsdayTV's Elisa DiStefano and Newsday lifestyle editor Meghan Giannotta have your inside look at Newsday's summer FunBook. Credit: Newsday/Howard Schnapp, Kendall Rodriguez, Drew Singh; Anthony Florio, Randee Daddona, Morgan Campbell, Debbie Egan-Chin

Get ready for sun and fun with NewsdayTV's summer FunBook special! From celebrating America's 250th birthday to a new ride at Adventureland, NewsdayTV's Elisa DiStefano and Newsday lifestyle editor Meghan Giannotta have your inside look at Newsday's summer FunBook.

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