BILLINGS, Mont. -- Montana environmental regulators have asked Exxon Mobil to justify its estimate for how much oil spilled into the Yellowstone River, citing the company's changing timeline on how long it took to stop a leaking pipeline.The Texas-based company estimates between 31,500 and 42,000 gallons of crude flowed into the Yellowstone near Laurel on July 1, fouling the shoreline and backwaters along dozens of miles of the scenic river.Read More... More on Environment