WASHINGTON (AP) — Government investigators have found crude oil being transported from North Dakota’s Bakken region was misclassified in samples taken from 11 out of 18 truck shipments en route to rail loading stations, federal transportation officials said Tuesday.

Hazardous materials shipments are supposed to be classified into one of nine categories depending on the risk involved. If the materials are misclassified, they could wind up being shipped in less protective rail tank cars and emergency personnel might follow the wrong protocols when responding to a spill.