Crude oil coming out of the Bakken formation in North Dakota might be more flammable than previously thought, U.S. officials said.

Rail derailments have become a somewhat frequent event in North Dakota. On Monday, the latest accident set off a series of explosions and left 21 railcars ablaze. Residents of Casselton, N.D. were asked to evacuate early Tuesday morning.

{mosads}The recent derailment is the fourth in North America over a span of six months, prompting the Pipeline and Hazardous Materials Safety Administration to issue a safety alert on Thursday.

The alert warned the public, emergency responders and carriers that Bakken crude oil might be more likely to set off an explosion than other types of crude.

The incidents indicate “that the type of crude oil being transported from the Bakken region may be more flammable than traditional heavy crude oil,” the safety administration said in the statement on Thursday.

The “light sweet crude oil” coming from the Bakken region has a burning point below 73 degrees Fahrenheit, officials warned.

“This means the materials pose significant fire risk if released from the package in an accident,” the safety administration said.