– Following a string of explosive accidents, federal officials said Thursday that crude oil being shipped by rail from the Northern Plains across the United States and Canada may be more flammable than other forms of oil.

A safety alert issued by the U.S. Department of Transportation warns the public, emergency responders and shippers about the potential high volatility of crude from the Bakken oil patch. The sprawling oil shale reserve has made eastern Montana and western North Dakota the nation’s second-largest oil producer behind Texas.

Thursday’s announcement declares that the Bakken’s light crude oil — which is often transported through Minnesota — may be different from traditional heavy crudes because it is prone to ignite at a lower temperature. Experts say lighter crudes, which contain more natural gas, have a much lower “flash point,” the temperature at which vapors given off by the oil can ignite.

The oil boom in the Bakken has reduced the nation’s reliance on imported oil, but it also has increased public safety concerns in communities bisected by rail lines. An estimated 11 to 12 crude oil trains depart daily from the oil region. Lacking sufficient pipelines, 69 percent of the state’s oil is shipped to market by rail. The main railroads, BNSF and Canadian Pacific, have tracks through the Twin Cities.

Minnesota doesn’t keep track of rail use or what trains are carrying. But Canadian Pacific and BNSF have boasted of their growing oil cargoes. In 2012, Canadian Pacific transported 53,500 cars of crude. It expects to move 140,000 to 210,000 by 2016.

That growth has echoed a national trend. The amount of U.S. oil moving by rail has spiked since 2009, from just more than 10,000 tanker cars to a projected 400,000 cars in 2013.

Thursday’s warning comes after a huge explosion on Monday caused by a crude train derailment near Casselton, N.D. No one was hurt, but worries about toxic fumes prompted the evacuation of hundreds of residents.

In July, 47 people were killed in Lac-Megantic, Quebec, when a train carrying Bakken crude derailed. Another oil train from North Dakota derailed and exploded in Alabama in November, causing no deaths but releasing an estimated 749,000 gallons of oil from 26 tanker cars.

By comparison, there was no fire when 10,000 gallons of oil that originated outside the Bakken region leaked after a Canadian Pacific Railway derailment in Minnesota last March. Cleanup crews were able to scoop up much of the spilled crude, which the railway said came from western Canada.

Whether the government’s response will help stave off another accident is uncertain. While safety advocates welcomed the move, others said it didn’t offer new information.

“It doesn’t take a rocket scientist to figure out that Bakken oil is a high-quality crude with a lower flash point — that’s what makes it a desired commodity for all these coastal refineries,” said Ron Ness, president of the North Dakota Petroleum Council, a Bismarck-based group that represents hundreds of oil industry companies.

The American Petroleum Institute, the oil industry trade group, pledged to review the findings and said it is working with regulators and the rail industry to ensure the safety of oil trains. “Any fuel or flammable product shipped in tank cars is held to the highest standard of safety, but it is still possible that rail accidents caused by human error, track defects, trains running into each other at very high speeds, or other rail issues could cause it to ignite,” the institute said.

Thursday’s safety alert resulted in part from results of preliminary tests on Bakken oil to determine just how dangerous it is, said Jeannie Shiffer with the Department of Transportation’s Pipeline and Hazardous Material Safety Administration.

Shiffer said knowing the volatility of the oil is crucial so that it can be properly handled during shipping. “The material must be properly classified at the beginning of the process. That determines everything,” she said.

The issue of volatility is particularly important for firefighters and other emergency responders who have to deal with accidents like the one in Casselton, said Fred Millar, a rail safety consultant in Virginia.

He said the dangers of crude have long been underappreciated, and need to be communicated to the hundreds of counties and cities across the United States that have seen a surge in crude oil trains.

After the Lac-Megantic crash, federal officials issued an advisory for companies to properly classify their crude oil according to a scale that ranks hazardous materials as a great danger, medium danger or minor danger. Officials have now gone a step farther, declaring that the Bakken’s light oil — extracted from shale formations through the drilling process known as hydraulic fracturing, or “fracking” — may be different.

But Larry Bierlein, an attorney for the Association of Hazmat Shippers, said the public would be better served by the government adopting a long-delayed proposal to improve railway cars.

North Dakota regulators had said last month that they were considering crafting a report to disprove that hauling the state’s crude by rail is dangerously explosive. On Thursday, officials said those plans had been dropped in the aftermath of the Casselton derailment.

Staff writer Jenna Ross contributed to this report.