Boren then "attempted to put the animal down himself," but one of the rounds he fired struck his patrol car instead, Cullimore said.
Cullimore said he did not know if the dog was injured before Boren tried to kill it, or if Boren ultimately was able to shoot the dog.
Cullimore said he received the disciplinary report regarding the incident from Utah County during Boren's background check and spoke with Boren about the shooting.
"It was something I believed was fairly minor and in the past and had been handled appropriately," Cullimore said.
Boren, 34, on Jan. 16 was found dead at his Spanish Fork home of a self-inflicted gunshot wound. Also shot to death were his 34-year-old wife Kelly; their two children, Joshua, 7, and Haley, 5; and Kelly's 55-year-old mother, Marie King.
State court records do not reveal that Joshua Boren had a history of violent crimes or domestic abuse, and police have said they were never called to the home on a complaint during the four years or so the family lived in Spanish Fork. However, Kelly Boren's friends have said Kelly told them her husband was abusive.
The couple had separated a couple of months before the family was killed.
Earlier on the day of the shootings, Kelly sent her friend RiRi Whiting a text message that said she'd decided to get a divorce, and that she couldn't stand Joshua's rage anymore.
Whiting texted back, "Rage?"
Kelly told her she'd explain later that night at the gym. But she never made it.
Joshua Boren didn't show up for work, either, and when Spanish Fork police arrived at the couple's 37 N. 630 West home at about 11 p.m. on Jan. 16, they found everyone dead.
Boren was hired by the Lindon police department on Oct. 14. Prior to that, he worked for about seven years as a corrections officer for the Utah County sheriff's office.