DALLAS (AP) — A Texas prisoner accused of smothering to death 22 older women in their Dallas-area homes so he could Arvin Robertssteal jewelry and other valuables was stabbed by his cellmate in an attack last month that killed him, authorities said.

The inmate accused of killing Billy Chemirmir, 50, on Sept. 19 at a rural East Texas prison was Wyatt Busby, 39, according to a Monday report from the Office of the Inspector General, which is investigating the killing. The report says Chemirmir, who was serving life in prison after being convicted last year in the slayings of two of the women, was “stabbed/beaten to death.”

A custodial death report filed with the attorney general’s office last week said that Chemirmir was killed with a “knife/edged instrument.” That report said that the means of death was blunt force trauma, but that the cause of death was pending.

Chemirimir was killed in the Coffield Unit about two weeks after the Texas Department of Criminal Justice placed its 100 prisons on a rare statewide lockdown because of a rise in the number of killings inside the facilities. While most prisons had resumed normal operations by Wednesday, the Coffield Unit remained on lockdown.

Chemirmir was arrested in 2018 after a 91-year-old woman survived an attack and told police a man had forced his way into her apartment at an independent living community for seniors, tried to smother her with a pillow and took her jewelry.

Police said they found Chemirmir the following day in the parking lot of his apartment complex holding jewelry and cash, having just thrown away a large red jewelry box. Documents in the jewelry box led them to the home of Lu Thi Harris, 81, who was dead in her bedroom.

Chemirmir was convicted of capital murder in the death of Harris and in a later trial in the killing of Mary Brooks, 87. Chemirmir, who maintained his innocence, was serving two sentences of life without the possibility of parole.

After Chemirmir’s arrest, police across the Dallas area began reexamining deaths, and the charges against him grew. He was eventually indicted on 22 capital murder charges.