The agency said in a news release Monday that an autopsy showed 48-year-old Dennis Marx died from multiple gunshot wounds suffered during a shootout with deputies Friday.
Forsyth County Sheriff Duane Piper asked the GBI to do a use of force investigation. The findings will be presented to District Attorney Penny Penn.
The courthouse reopened Monday after repairs were made over the weekend to walls and windows struck by bullets.
Authorities have said Marx had an assault rifle, two handguns, explosives and supplies to take hostages.
The deputy was shot in the leg and was in stable condition over the weekend.
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