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An Exxon Mobil pipeline that ruptured, leaking oil into Yellowstone River, may have sometimes carried a heavier and more toxic form of crude than initially thought, federal regulators said on Thursday.

The U.S. Transportation Department’s Pipeline and Hazardous Materials Safety Administration spokeswoman Patricia Klinger said her office had learned that the pipeline may have been used to carry heavier crude.

“I just found out that apparently, and the regional folks just found out, there is an interconnect on the pipeline that possibly does carry some oil out of Canada,” she said in response to a question about tar sands crude in the pipeline.

That a pipeline thought to transport only “sweet,” low sulfur crude could have carried so-called tar sands crude from Canada raised concerns by health and environmental officials, even as Exxon officials said the heavier oil was not flowing through the Silvertip pipeline when it broke on July 1.

“The actual crude in the line at the point of the incident was a blend of crudes from Wyoming,” Exxon spokesman George Pietrogallo told Reuters in an email on Thursday.

Exxon was responding to a question about whether tar sands crude had ever flowed in the pipeline. Almost all the oil produced in Canada’s Alberta fields is from tar sands.

The chemistry of tar sands oil, derived from tar sands or bitumen and sweet crude is significantly different, said Ronald Kendall, head of the environmental toxicology department at Texas Tech University.