Sheriff Larry Williams told The Times and Democrat of Orangeburg that investigators are trying to figure out how the much more volatile gasoline got into the diesel tank.
The explosion happened Monday as three people tried to fill a logging truck with fuel at a Norway home.
Authorities say the men were using a homemade fuel pump system powered by jumper cables connected to the truck's batteries when one of them opened the tank's cap. The survivor told investigators he saw a blue flame shoot up and the tank exploded.
Sheriff Williams says deputies may never know how much gasoline was mixed with the diesel fuel.