WASHINGTON (AP) — Federal inspectors found a problem in an oil pipeline a month before it ruptured in a Montana river, but it was not significant enough to force a shutdown, the government’s top pipeline regulator said Wednesday.

Cynthia Quarterman, administrator of the U.S. Pipeline and Hazardous Materials Safety Administration, told a Senate committee that the problem in the Exxon Mobil pipeline was discovered in early June, nearly a month before the pipeline ruptured on July 1, spilling an estimated 42,000 gallons of crude into the Yellowstone River.

Quarterman declined to offer specifics about what she called an “anomaly” but said officials believed it was not significant enough to require repairs. It wasn’t clear whether the problem detected in June had any role in the rupture.

“Unfortunately, I don’t think that our pipeline inspector thought he had the authority to shut down the pipeline,” she told a Senate Environment subcommittee.

Agency officials said a review of pipeline records conducted in June revealed that the half-inch thick steel pipeline had about 20 percent external corrosion, based on inspections in 2004 and 2009. The review was part of a larger examination of pipeline records in response to high water flows throughout the Mountain West because of an unusually high spring snow melt.

The rupture in the 12-inch diameter pipeline, which had been buried below the riverbed, occurred as the Yellowstone River flooded following heavy rains. Debris, including trees, was floating in the river at the time of the accident.

Sen. Max Baucus, D-Mont., said he was disappointed in the federal response.

“To be honest, ma’am, it sounds like you’re not on top of this,” he told Quarterman.

Baucus also scolded Gary Pruessing, president of Exxon Mobil Pipeline Co, saying that the oil executive appeared to waver on initial promises to make land owners near the spill “whole” following the spill and cleanup.

The company takes full responsibility for the incident and the cleanup, “and we pledge to satisfy all legitimate claims,” Pruessing said.

Baucus said there was plenty of blame to go around.

“The company made a mistake. It was wrong about the integrity of the pipeline,” he said. The pipeline agency also was wrong, Baucus added: “It made a mistake about the integrity of the pipeline.”

The cause of the spill remains under investigation, but early signs indicate that the pipeline was completely severed in the accident, according to the Montana Department of Environmental Quality. The findings suggest that the pipeline was undercut by the river and broke, rather than springing a leak due to corrosion in the line.

It will probably be August or September before water levels in the river are low enough to raise the section of damaged pipe responsible for the spill, Quarterman said.

It could take another two months after that before investigators identify a cause, and Quarterman said her agency won’t know for certain how large the leak was until it examines records at the oil company’s control room in Houston.

Officials in Laurel, Mont., near the site of the spill, raised questions last year about erosion along the riverbank threatening Exxon Mobil’s Silvertip pipeline. 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 a day later, following a review of its safety record.

“At the time this incident occurred, we did not have any outstanding issues from a regulatory standpoint on this pipeline,” Pruessing said.

Copyright 2011 The Associated Press.