BILLINGS, Mont. - Following a string of explosive accidents, federal officials say crude oil being shipped by rail from the Northern Plains across the U.S. and Canada may be more flammable than traditional 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 is fuelling the surging industry in eastern Montana and western North Dakota, which is now the nation's second-largest oil producer behind Texas.

Thursday's announcement from officials declares that the Bakken's light, sweet crude oil 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 vapours given off by the oil can ignite.

The government'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 from the small eastern North Dakota town.

The oil boom in the Bakken has reduced the nation's reliance on imported oil and brought thousands of jobs to the region. But as companies increasingly rely on trains instead of pipelines to get that oil to lucrative coastal markets, public safety in communities bisected by rail lines has become a major concern.

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 to the latest derailment will help stave off another accident is uncertain. While safety advocates welcomed the move, others said the warning 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.

Ness added that companies shipping oil from the Bakken already were adhering to federal regulations.

Casselton Mayor Ed McConnell agreed that there was no surprise in the federal government's assessment that Bakken crude may be more volatile.

"The important thing and the intent here is to keep pressure on the federal and state government to make things safer," he added.

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

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 U.S. that have seen a surge in crude oil trains. He added that railroads should consider routing the Bakken oil trains around highly populated areas.

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 further, declaring that the Bakken's light oil — extracted from shale formations through the drilling process known as hydraulic fracturing, or "fracking" — may be different.

Kenneth Medlock, senior director at the Center for Energy Studies at Rice University in Houston, noted that the volatility of crude varies from one oil field to the next and is driven largely by how heavy it is.

Given that oil is typically shipped in the same type of tank car no matter its volatility, Larry Bierlein, an attorney for the Association of Hazmat Shippers, questioned the importance of Thursday's announcement. Flaws that render the most commonly used tank cars prone to rupture have been known for more than two decades.

Bierlein said the public would be better served by the government adopting a long-delayed proposal to improve those cars, known as DOT-111s.

"They have lost track of where safety is," he said of the Department of Transportation.

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, state officials said those plans had been dropped in the aftermath of the Casselton derailment.

The state's oil production is now pegged at about 1 million barrels daily, and the state's sweet crude is increasingly moving by rail to feed refineries on the East, West and Gulf coasts.

State officials said the pace of production likely won't be stunted by Monday's accident.

___

James MacPherson reported from Bismarck, N.D.

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  • Firefighters continue to water smoldering rubble Sunday, July 7, 2013 in Lac Megantic, Quebec. A runaway train derailed Saturday igniting tanker cars carrying crude oil.

  • The downtown core lays in ruins as firefighters continue to water smoldering rubble Sunday, July 7, 2013 in Lac Megantic, Quebec. A runaway train derailed Saturday igniting tanker cars carrying crude oil.

  • The remains of tanker cars lie burnt out in the downtown core Sunday, July 7, 2013, in Lac Megantic, Quebec. A runaway train derailed Saturday igniting tanker cars carrying crude oil and devastated the town.

  • The downtown core lays in ruins as firefighters continue to water smoldering rubble Sunday, July 7, 2013 in Lac Megantic, Quebec. A runaway train derailed Saturday igniting tanker cars carrying crude oil.

  • The downtown core lays in ruins as firefighters continue to water smoldering rubble Sunday, July 7, 2013, in Lac Megantic, Quebec. A runaway train derailed Saturday igniting tanker cars carrying crude oil.

  • The downtown core lays in ruins as fire fighters continue to water smouldering rubble Sunday, July 7, 2013 in Lac Megantic, Quebec after a train derailed ignited tanker cars carrying crude oil.

  • The downtown core lays in ruins as firefighters continue to water smoldering rubble Sunday, July 7, 2013, in Lac Megantic, Quebec. A runaway train derailed Saturday igniting tanker cars carrying crude oil.()

  • The downtown core lays in ruins as firefighters continue to water smoldering rubble Sunday, July 7, 2013 in Lac Megantic, Quebec. A runaway train derailed Saturday igniting tanker cars carrying crude oil.

  • The downtown core lays in ruins as fire fighters continue to water smoldering rubble Sunday, July 7, 2013 in Lac Megantic, Quebec after a train derailed ignited tanker cars carrying crude oil.

  • Burnt out oil tank cars and the destroyed downtown core lays in ruins as firefighters continue to water smouldering rubble Sunday, July 7, 2013, in Lac Megantic, Quebec.

  • The downtown core lays in ruins as firefighters continue to water smoldering rubble Sunday, July 7, 2013, in Lac Megantic, Quebec, after a train derailed ignited tanker cars carrying crude oil. The runaway train derailed, causing explosions and fires that destroyed the downtown district.

  • The downtown core lays in ruins as firefighters continue to water smoldering rubble Sunday, July 7, 2013, in Lac Megantic, Quebec, after a train derailed ignited tanker cars carrying crude oil.

  • Burnt out oil tank cars and the destroyed downtown core lays in ruins as firefighters continue to water smouldering rubble Sunday, July 7, 2013, in Lac Megantic, Quebec. A runaway train derailed igniting tanker cars carrying crude oil on Saturday.

  • An aerial view of a fire in the town of Lac-Megantic is seen from a Sûreté du Québec helicopter Saturday, July 6, 2014 following a train derailment the sparked several explosions in Lac Megantic, Quebec.

  • Smoke rises from railway cars that were carrying crude oil after derailing in downtown Lac Megantic, Quebec, Canada, Saturday, July 6, 2013.

  • Smoke rises from railway cars that were carrying crude oil after derailing in downtown Lac Megantic, Quebec, Canada, Saturday, July 6, 2013.

  • Smoke rises from railway cars that were carrying crude oil after derailing in downtown Lac Megantic, Quebec, Canada, Saturday, July 6, 2013.

  • Smoke rises from railway cars that were carrying crude oil after derailing in downtown Lac Megantic, Quebec, Canada, Saturday, July 6, 2013.

  • Smoke rises from railway cars that were carrying crude oil after derailing in downtown Lac Megantic, Que., Saturday, July 6, 2013.

  • Smoke rises from railway cars that were carrying crude oil after derailing in downtown Lac Megantic, Quebec, Canada, Saturday, July 6, 2013.

  • This aerial photo shows a fire in the town of Lac-Megantic as seen from a Sûreté du Québec helicopter Saturday, July 6, 2013 following a train derailment that sparked several explosions in Lac Megantic, Quebec.

  • MORE LAC MEGANTIC PHOTOS

  • A levelled building is seen in the foreground the day after a train derailed causing explosions of railway cars carrying crude oil Sunday, July 7, 2013 in Lac Megantic, Que.

  • Canadian Prime Minister Harper, right, surveys the scene Sunday, July 7, 2013 in Lac Megantic, Quebec, as firefighters continue to spray derailed tanker cars. A runaway train derailed Saturday causing explosions of railway cars carrying crude oil and destroyed part of the downtown area of Lac Megantic.

  • Prime Minister Harper, centre, talks with firefighters Sunday, July 7, 2013 in Lac Megantic, Quebec. A runaway train derailed Saturday exploding tanker cars carrying crude oil and destroying part of the downtown area of Lac Megantic.

  • Prime Minister Stepen Harperwaves to people as he visits the site Sunday, July 7, 2013 in Lac Megantic, Quebec, where a runaway train derailed Saturday. Tanker cars carrying oil exploded after the derailment and destroyed the business district in Lac Megantic.

  • Firefighters keep watering railway cars the day after a train derailed causing explosions of railway cars carrying crude oil Sunday, July 7, 2013 in Lac Megantic, Que.

  • The downtown of Lac Megantic, Que. is seen Sunday, July 7, 2013, the day after a train derailed causing explosions of railway cars carrying crude oil.

  • Burnt buildings are seen following a train derailment causing explosions of railway cars carrying crude oil Sunday, July 7, 2013 in Lac Megantic, Que.

  • Workers inspect a railway car that was part of a convoy of railway cars carrying crude oil that derailed Saturday causing explosions Sunday, July 7, 2013 in Lac Megantic, Que.

  • Workers inspect a railway car that was part of a convoy of railway cars carrying crude oil that derailed Saturday causing explosions Sunday, July 7, 2013 in Lac Megantic, Que.

  • A fire keeps burns after railway cars that were carrying crude oil derailed in downtown Lac Megantic, Quebec, Saturday, July 6, 2013.

  • Firefighters douse a blaze after a freight train loaded with oil derailed in Lac Megantic in Canada's Quebec province on June 6, 2013, sparking explosions that engulfed about 30 buildings in fire. At least one person has been killed.

  • Smoke rises in the background as Quebec Premier Pauline Marois speaks to reporters after a freight train loaded with oil derailed in Lac Megantic in Canada's Quebec province on June 6, 2013, sparking explosions that engulfed about 30 buildings in fire. At least one person has been killed.

  • Residents watch rising smoke after a freight train loaded with oil derailed in Lac Megantic in Canada's Quebec province on June 6, 2013, sparking explosions that engulfed about 30 buildings in fire. At least one person has been killed.