SALT LAKE CITY (AP) -

Authorities say a Utah police officer used his service weapon when he killed his wife, two children and his mother-in-law before turning the gun on himself in January.

Spanish Fork City Police Lt. Matt Johnson says the agency's investigation was unable to determine Lindon police officer Joshua Boren's exact motive. But he says investigators did learn the 34-year-old Boren and his wife were having marital problems.

Toxicology reports from the autopsy show Boren had no drugs or alcohol in his system. Johnson also says Boren had no history of violence or mental illness.

Police say the state medical examiner confirmed what investigators believed: Boren shot his wife, 32-year-old Kelly Boren; his 55-year-old mother-in-law, Marie King; and his two children, 7-year-old Joshua "Jaden" and 5-year-old Haley, before killing himself.

The shooting happened at the family's home in Spanish Fork, about 50 miles south of Salt Lake City.