BAGHDAD, Iraq (CNN) -- A vehicle thought to be packed with explosives blew up Wednesday as it passed a Baghdad hotel, killing the driver and at least two bystanders, a U.S. military official said. The explosion occurred at dawn near the Shaheen Hotel in the Karada neighborhood of central Baghdad. The bystanders were a South African contractor and an Iraqi civilian. At least four Iraqis were wounded, the U.S. official said. Iraqi police said explosives may have been packed inside an ambulance or a vehicle made to look like an ambulance. It is not clear if the blast was a suicide bombing or if the explosives detonated accidentally. copyright� CNN