An Oklahoma sheriff's deputy involved in a sting operation with a volunteer deputy who fatally shot an unarmed man has resigned from the agency.

Tulsa County Sheriff's Office spokesman Justin Green said Wednesday that deputy Michael Huckeby submitted his resignation effective Thursday. Green gave no other details.

Huckeby was involved in an April gun-sales sting with reserve deputy Robert Bates, who fatally shot Eric Harris during the operation. A video of the shooting shows Huckeby placing his knee on Harris' head as he lay on the ground after being shot.

Bates has said his mistook his hand gun for his stun gun. He's pleaded not guilty to second-degree manslaughter.

A grand jury is investigating allegations of wrongdoing in the sheriff's office.