Stephen Grant's father, William "Al" Grant, committed suicide Friday, said the St. Clair County Medical Examiner's Office.
The office said it looks like an apparent self-inflicted gunshot to the head.
William Grant was taken to Port Huron Hospital, where he was pronounced dead. The autopsy will be performed at the hospital.
He was 66 years old.
Stephen Grant was convicted of murdering and dismembering his wife, Tara Grant, in 2007.