(Reuters) — The Obama administration weighed national standards to control explosive gas in oil trains last year but rejected the move, deciding instead to leave new rules to North Dakota alone.

Current and former administration officials told Reuters that they were unsure of federal jurisdiction to force the energy industry to drain volatile gas from crude oil originating in North Dakota's fields.

Instead, they opted to back North Dakota's effort to remove the cocktail of explosive gas — known in the industry as 'light ends' — and rely on the state to contain the risk.

North Dakota's regulations come into force next month.

The administration's internal debate shows that concern about the risks associated with oil trains reached the upper level of the White House. But the administration balked at addressing the problem in new regulations governing crude oil trains that it is preparing to introduce this spring.

A growing number of safety advocates say those rules are insufficient to regulate a product that is hauled along thousands of miles of track and across many state lines.

"These trains are going all across the country, so it absolutely has to be the feds who are in charge," said Karen Darch, mayor of Barrington, Illinois, where several oil and ethanol trains pass through her town weekly.

Last September, Transportation Secretary Anthony Foxx took his concerns about Bakken fuel to the White House and sought advice on what to do about the danger of light ends, according to sources familiar with the meeting.

By then, Mr. Foxx had spent more than 12 months weighing safety measures that would prevent oil train derailments from becoming fiery disasters like the 2013 Lac Mégantic tragedy in Ontario in which 47 people were killed by a runaway Bakken train delivery.

The Transportation Department was warning that Bakken fuel was uncommonly volatile and explosion-prone. Mr. Foxx's agency conceived an oil train safety plan in July with an array of measures that aimed to make sure oil train cargo moved safely on the tracks.

Tankers would have toughened shells. Oil train deliveries would slow down. Advanced braking systems would be adopted.

But the rule would do nothing to limit volatile gas.

Mr. Foxx brought his concerns about the unresolved issue of dangerous gas, commonly measured as vapor pressure, and his agency's limited power to curtail the problem to President Barack Obama's chief of staff, Denis McDonough. The administration decided to just let the existing oil train safety plan take root.

"The department had already identified issues with the characteristics of the crude oil, including vapor pressure, and had developed potential strategies related to the overall improvement and safety of the transport of the product and how the industry could treat it," a White House official told Reuters.

But ultimately, the administration decided to support North Dakota's plan to limit vapor pressure — a measure that was just taking shape at the time the officials met.

"Following the meeting, the Department of Transportation supported North Dakota on treatment of crude oil in the field," the White House official said.

That approach is not good enough for many critics.

Sen. Charles Schumer, D-N.Y., warned this week that oil train "disasters" could continue "until the stability of the crude being loaded into the tank cars themselves is improved."

Of the roughly 1.2 million barrels of crude oil produced in North Dakota daily, more than 60% of that fuel reaches refineries by rail, typically in 100-tanker unit trains that can stretch a mile long.

A large share of that fuel moves through New York on the way to refineries in the mid-Atlantic.

In a letter to Secretary Foxx and Energy Secretary Ernest Moniz, Sen. Schumer encouraged the officials "to work together to develop new regulations that would require the stabilization of crude oil prior to shipment."

An Energy Department official said the agency is in the early stages of developing a report on Bakken crude dangers that "may be of use to the Department of Transportation, which has regulatory authority over the transport of crude oil."