"He started to argue with her; at one point, she says he 'poked' her in the nose," Jackson said.
Diem's stepfather, 79-year-old Melvin Rowley saw this and tried to intervene, Jackson said.
The parents "maneuvered" Diem into his bedroom and shut the door, Jackson said. Diem kicked a wall and broke the door open; he then hit Rowley several times in the face, Jackson said. Rowley fell to the ground, and Diem climbed on top of him and continued to strike him as his mother tried to stop him, Jackson said.
Diem then took his backpack, left the house and rode away on his bicycle, Jackson said. Rowley told his wife he was fine, so she followed Diem out of the house and told a concerned neighbor to call 911.
When Rowley's wife returned to the house, she found Rowley unconscious and not breathing. She tried to perform CPR with the help of the neighbor; police officers and medics who arrived at the house continued efforts to revive him. Rowley was flown to Utah Valley Medical Center, where he died about 1:30 a.m.
Officers found Diem about midnight near his neighborhood. He was arrested and booked into the Utah County jail on suspicion of manslaughter, Jackson said.