GIBSONIA, Pa.—The family of a western Pennsylvania man who's accused of stabbing his elderly grandmother to death during breakfast says they're speaking out about his mental illness.

KDKA-TV reports Tuesday that 26-year-old Levi Daniel Staver struggled with Asperger's Syndrome and schizophrenia.

Ruth Johnson tells KDKA ( http://cbsloc.al/ZHcxuC) that Staver believed his grandmother had tried to attack him, though that never happened. He's been charged with criminal homicide in the Feb. 19 death of 76-year-old Constance Johnston.

Police say Staver came upstairs from the basement and stabbed Johnson in the back with a hunting knife.

Authorities allege that Staver told police he was sitting at his computer when "the archangel" told him to "kill the witch."