Detective Chief Inspector Grant Mallon, the senior investigating officer into the death, said two men were incapacitated and detained at the scene of Rigby's death.
He is the only person with the power to grant clemency to stay the execution and commute Williams' four death sentences to life imprisonment with no parole.
Grant was jailed at the High Court in Edinburgh in July after he admitted causing Mr Quinnin's death by driving carelessly and while over the drink drive limit.
Grant was jailed at the High Court in Edinburgh in July after he admitted causing Mr Quinnin's death on the A75 near Kirkcowan in June 2011 by driving carelessly and while over the drink drive limit.