An Oklahoma father is believed to have fatally shot his wife and three of their children in a grisly murder-sucide that authorities labeled a "mystery," according to a report Tuesday.

Police officers found the five family members dead in their home in the Czech Hall Estates neighborhood of Oklahoma City after getting a call around 9:35 a.m. Monday.

An unidentified 10-year-old boy discovered the bodies, the Oklahoman newspaper reported, citing a law enforcement official who disclosed details from a search warrant.

A gun was lying on top of the dead father, identified as Jonathon Candy, 42.

Investigators believe Candy killed his wife, Lindsay Candy, 39, and sons Dylan, 18, Ethan, 14, and Lucas, 12, before fatally shooting himself.

A fourth child in the home was unharmed and was placed in the care of relatives.

"It remains a mystery as to what caused Mr. Candy to murder his wife and three of his children," police said in a prepared statement Tuesday, according to the Oklahoman.

The superintendent of the Mustang school district said in an email to parents Monday that the two of the slain children were students and the third graduated last year.

"We are shocked, and our hearts are broken; this tragedy simply defies understanding," superintendent Charles Bradley wrote. 

If you or someone you know needs help, the national suicide and crisis lifeline in the U.S. is available by calling or texting 988. There is also an online chat at 988lifeline.org