Ex-death row inmate Dennis Stanworth -- who proclaimed, "I plead guilty to everything" at his murder arraignment last year -- was found mentally incompetent Tuesday to stand trial for allegedly slaying his mother in Vallejo in 2012.

Solano Superior Court Judge Robert Bowers issued the ruling after prosecutors dropped their request to hold a jury trial to determine Stanworth's competency.

Prosecutors sought the competency trial in December after two court-appointed doctors found that Stanworth, 72, could not rationally assist in his defense. According to court records, Stanworth -- who has appeared confused and feeble at his court hearings -- was diagnosed with dementia and possible Alzheimer's disease connected to a stroke that occurred before his arrest.

Stanworth was convicted in 1966 of murdering two Pinole girls, one of whom he raped after shooting them both in the head.

Despite pleading for death, he was spared the gas chamber in 1972 after the California Supreme Court ruled capital punishment unconstitutional. His sentence was reduced to life in prison, but he was paroled in 1990 and required to register as a sex offender.

Stanworth was arrested on Jan. 9, 2013, after he called Vallejo police and confessed to killing his mother, 90-year-old Nellie Stanworth, at his Hiddenbrooke-area home. Two days later at his arraignment, he proclaimed, "It's the third time" and "I plead guilty" before his attorney could finish seeking a delay in the case. He later entered a not guilty plea.

According to testimony at his preliminary hearing in August, Stanworth told police he hit his mother in the head with a brick and slashed her throat before burying her in his yard.

He later stuffed her decomposing body into a garbage can filled with dirt, police said. Stanworth told police he killed his ailing mother because he was concerned about who would take care of her if he died before she did.

Stanworth, who is being housed at the state-run California Medical Facility in Vacaville, was not present at Tuesday's hearing in Bowers' Fairfield courtroom. A hearing to determine his placement in a state hospital was scheduled for April 16.

Prosecutors have voiced concerns about Stanworth's mental condition worsening beyond repair, in which case he could be released back into society after being held for the maximum of three years in the state's competency training program.