BILLINGS, Mont. (AP) — Federal regulators offered more details on testing requirements for oil transported by rail on Thursday and warned companies against skirting the rules after a spate of explosions caused by crude train derailments in the U.S. and Canada.

The new order from the U.S. Department of Transportation builds on a Feb. 25 declaration that the industry’s unsafe handling practices have made crude shipments an imminent hazard to public safety and the environment.