Coulter was arrested on suspicion of fatally shooting 18-year-old Monique Cota in his bedroom on Oct. 15, 2011.
Investigators say Coulter had assaulted Cota in an earlier incident.
Coulter claimed the shooting was an accident that occurred as he was inspecting an AR-15 rifle. He said he pulled the charging handle and bolt back several times and showed Monique the rifle was empty, but the rifle discharged striking Monique in the head, prosecutors said.
Police say Coulter's sister then hid guns that were in the house and drove him to the motel room in Mesa.
Coulter was later arrested at the hotel.
Police learned from the victim's family that Monique and Coulter began dating in 2009 and occasionally lived together at Coulter's parent's home. Their relationship was volatile and both had been arrested for a previous domestic violence dispute that broke out at the McKemy Avenue home, the Maricopa County Attorney's Office said.
Family members said that Coulter had been physically abusive towards Monique and that she planned to end their relationship, prosecutors said.
Several days before she was killed, Monique told her sister that she had become engaged to a childhood friend who lived in California and that she was planning to relocate there and live with him, prosecutors said.
Copyright 2013 Associated Press. All rights reserved. CBS 5 (KPHO Broadcasting Corporation) contributed to this report.