Oklahoma has executed a death row inmate nearly 20 years after he killed his ex-girlfriend and her infant daughter and set them on fire.

Raymond Eugene Johnson, 52, was executed by lethal injection and pronounced dead at 10:12 a.m. CT on Thursday, May 14. He was convicted of the brutal 2007 murders of 24-year-old Brooke Whitaker and her 7-month-old baby girl Kya. Johnson repeatedly hit Whitaker with a claw hammer before setting her and her Tulsa home on fire.

Kya was killed in the blaze and Whitaker − a mother of four − later died at a hospital of severe burn wounds and head injuries.

"Raymond Johnson is a heinous murderer who violently attacked and tortured Brooke Whitaker before ultimately setting her and her infant daughter on fire," Oklahoma Attorney General Gentner Drummond said in a statement last month. "Johnson has never taken full accountability for these unconscionable acts."

Johnson had been arguing that he deserved clemency from the death penalty, saying he was a changed man.