REDWOOD CITY (BCN) — A jury found a 51-year-old San Francisco resident guilty of first-degree murder Thursday for killing his girlfriend in San Bruno in 2013, prosecutors said.

The jury deliberated for less than three hours before finding Alberto Trejo guilty of all charges, including felony possession of a firearm by a felon, prosecutors said. Prosecutors said the murder charge includes an enhancement for use of a firearm.

Trejo and his girlfriend were living together with others in a San Bruno home when on Feb. 13, 2013, Trejo called another resident crying and saying his gun went off, according to prosecutors.

Trejo was busy with laundry when other residents came home and he told them he had an argument with his girlfriend and she left, according to prosecutors. His girlfriend was last seen that day.

Prosecutors said the victim’s mother reported her daughter’s disappearance a few days later.

The body was discovered April 26, 2013, at a Pacifica apartment complex. The cause of death was a single bullet to the back of the head, according to prosecutors.

Trejo faces a sentence of 91 years to life in state prison when he’s sentenced on Sept. 4. He remains in custody without bail.