BILLINGS, Mont. - 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, on Friday night, 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.

As residents along the river deal with an oil-smeared shoreline and workers clean up the mess, the accident has raised concerns about the impact that the season's floods may be having on the network of pipelines buried under riverbeds.

"It's too early to tell whether this is an isolated incident or there might be other types of increased damage or erosion based on a year of flooding," said Brigham McCown, a former federal pipeline safety official who now advises pipeline companies at a Dallas firm.

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.

The company said in a June 1 email - just a month before the spill - that the line was buried at least 12 feet beneath the riverbed, according to documents from the U.S. Department of Transportation, which oversees pipelines.

The documents also contained additional details that raised new questions about the company's response.

Exxon Mobil took almost an hour to fully seal the pipeline after the accident - nearly twice as long as it had publicly disclosed. The company said that did not change its estimate of how much crude entered the river.

"The best thing they could do at this point is be completely honest," said Montana Gov. Brian Schweitzer. "It is clear that their veracity has not been 100 percent to this point."

Company representatives initially said the spill lasted "at most" 30 minutes, and then later said workers began shutting down the line within six minutes of the break. On Tuesday, Pruessing said in response to a question from Schweitzer that it took 30 minutes to seal off all the valves needed to stop the flow of crude into the river.

DOT records indicate the pipeline was not fully shut down for 56 minutes after the break at 10:40 p.m. local time. Emergency responders at the National Response Center were notified of the spill at 12:19 a.m.

Federal regulations require that pipelines be buried more than 4 feet beneath the riverbed at stream crossings.

McCown said he believes most pipelines are buried near that depth, although there are some exceptions.

In normal weather conditions, about four feet below ground is a safe depth, but pipeline companies should be paying close attention to the safety of their networks given this year's unusual weather and record floods, he said.