The order, which requires shippers to test the crude oil coming from the Bakken region in Montana and North Dakota, warns against improperly classifying the oil before it is transported by train.

This is the fourth warning by the department on safety concerns over crude oil in the last seven months.

“Today we are raising the bar for shipping crude oil on behalf of the families and communities along rail lines nationwide — if you intend to move crude oil by rail, then you must test and classify the material appropriately,” said Transportation Secretary Anthony Foxx in a statement.

“And when you do ship it, you must follow the requirements for the two strongest safety packing groups. From emergency orders to voluntary agreements, we are using every tool at our disposal to ensure the safe transportation of crude.”

Effective immediately, those transporting crude must properly test the oil and classify it according to federal safety regulations.

Earlier this month, the Transportation Department hit three companies with notices of possible violations for improperly labeling crude coming from the Bakken. The fines totaled $93,000.

In January, officials warned that Bakken crude could pose a greater flammability risk than previously thought.

In the last seven months there have been roughly four derailments of trains carrying crude oil. The accidents prompted the Pipeline and Hazardous Materials Safety Administration to issue an alert in January warning the public, emergency responders and carriers that Bakken crude oil may be more likely to set off an explosion than other types of crude.