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.