According to the Associated Press, Exxon Mobil Co. had reassured federal regulators and officials from a Montana town since December that an oil pipeline beneath the Yellowstone River was safe, buried deep enough to avoid any accidental ruptures. Then the pipe failed, spilling an estimated 42,000 gallons into the flooded river. The cause of the accident remains under investigation, but the prevailing theory among officials and the company is that the raging Yellowstone eroded the riverbed and exposed the line to damaging rocks or debris. There is still no definitive word on how far downriver the spill could spread. Oil has fouled miles of the waterway that flows from the famed Yellowstone National Park, upriver from the spill, and across farmlands and prized fishing grounds, to North Dakota. Crude has been reported as far as 240 miles downstream, although most appears to be concentrated in the first 25 miles. Officials in Laurel, near the site of the spill, raised questions last year about erosion along the riverbank threatening the Exxon Mobil line. The company in December surveyed the pipe''s depth and said it was at least 5 to 8 feet beneath the riverbed. The line was temporarily shut down in May after Laurel officials again raised concerns that it could be at risk as the Yellowstone started to rise. The company restarted the line after a day, following a review of its safety record.