An 89-year-old man who fatally stabbed his grandson did so in self-defense, authorities in Putnam County said.

An investigation done by the sheriff's and State Attorney's offices determined Tuesday that Howard Grisham killed his grandson during an altercation in September.

Police said Jared Thorn, 22, lived with his grandfather in Crescent City. Thorn was charged with aggravated battery in June after he pushed Grisham's 75-year-old wife to the ground.

Grisham was taken to a hospital with injuries suffered during the fight and was later released. Thorn died from a stab wound to his stomach.

Grisham is a former Putnam County commissioner who served one four-year term before being voted out in 2000.