TULSA, Okla. (AP) - A former Oklahoma volunteer sheriff's deputy who said he mistook his handgun for his stun gun when he fatally shot an unarmed suspect last year has been sentenced to four years in prison.

A judge on Tuesday gave 74-year-old Robert Bates the maximum penalty recommended by jurors. The wealthy insurance executive was convicted of second-degree manslaughter last month.

Bates fatally shot Eric Harris while working with Tulsa County sheriff's deputies last year during an illegal gun sales sting. Harris was restrained and unarmed at the time.

After the shooting, an internal 2009 sheriff's office memo surfaced that questioned Bates' qualifications. Bates also was a close friend of then-Sheriff Stanley Glanz and had donated thousands of dollars in cash, vehicles and equipment to the sheriff's office.