Federal regulators have acknowledged the Exxon Mobil pipeline that recently ruptured under Montana’s Yellowstone River may have been carrying tar sands oil, a heavier, more toxic form of crude oil. Initially Exxon Mobil said the pipeline was only carrying oil from Montana and Wyoming, but on Thursday the company admitted the pipeline was also carrying oil from Canada, the home of the major tar sands deposits in Alberta. Meanwhile, a group of Democratic senators have called on the Department of State to conduct additional environmental reviews before approving a massive new tar sands oil pipeline known as Keystone XL that would stretch from Montana to Texas.