NASCAR star Jimmie Johnson's wife's parents and 11-year-old nephew died following a fatal shooting at their Oklahoma home on Monday ... and according to law enforcement, the tragedy is being investigated as a murder-suicide.

Muskogee Police tell TMZ Sports -- Chandra Janway's parents, Jack and Terry Janway, and nephew, Dalton Janway, were found dead shortly after 9 PM ... after receiving a 911 call from a woman who reported someone with a gun before hanging up the phone.

Cops say Terry is considered the suspect ... but the motive is unknown and currently under investigation. It's not clear if she's the one who made the 911 call.

Cops tell us they found a body laying in the hallway after arriving at the scene ... and heard another gunshot come from inside the home.

Cops searched the house and found two more dead bodies. All three died from gunshot wounds, police say.

Johnson -- a 7-time NASCAR Cup champion -- married Chandra in 2004 ... and they have two daughters together, Genevieve and Lydia.

Jimmie is currently slated to compete in NASCAR's Chicago street race this weekend.

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