The CSB confirmed that the truck driver killed at a nearby oasis was hit by a large piece of pipe from the NDK plant.
Investigators say a pressure vessel blew up, but they have not figured out the exact cause of the explosion. But they determined it was not triggered on purpose.
"It's almost impossible to do it intentionally. This particular incident, based on the characteristics, it certainly falls into that, that it just couldn't be intentionally done," said John Vorderbrueggen, U.S. Chemical Safety Board.
The CSB could take up to two years to complete its investigation.
