A man on death row in a California state prison died Wednesday, the California Department of Corrections and Rehabilitation announced.

Micky Ray Cage, 55, was convicted of murdering his estranged wife’s mother and her teenage brother in November 1998 in Moreno Valley.

During his trial, it was revealed that he had an abusive and controlling relationship with his estranged wife. When she took her children to Puerto Rico to stay with friends, Cage became enraged and obsessed with finding his children, court documents state.

Cage turned his anger toward his wife’s mother, arriving at her home in the night and shooting her in the head, shoulder and chest with a shotgun he concealed in a laundry basket. After killing her, he went upstairs to the bedroom of his wife’s 16-year-old brother and shot him in the chest at close range.

He was found guilty of first degree murder and sentenced to death in 2003. He appealed the conviction, but it was upheld by the California Supreme Court in 2015.

Cage remained in custody, but was never executed due to Gov. Gavin Newsom implementing a moratorium on the practice as one of his first acts in office.

He was found unresponsive in his cell by medical staff at Salinas Valley State Prison in Monterey County Wednesday morning.

His cause of death is under investigation by the Monterey County Coroner’s Office.

Cage was among the hundreds of people condemned to death in California. CDCR says there are currently 664 condemned people incarcerated in its prisons.