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BILLINGS, Mont. (AP) -- Delays in Exxon Mobil Corp. s response to a major pipeline break beneath Montana s Yellowstone River made the spill far worse than it otherwise would have been, federal regulators said.

Department of Transportation investigators examining the spill said pipeline controllers in Houston could have reduced the volume of the 1,500-barrel spill by about two-thirds if they isolated the rupture as soon as problems emerged.

Instead, crude drained from the severed, 12-inch pipeline for another 46 minutes before a remote control valve near the river was finally closed.

The July 1, 2011, spill fouled 70 miles of the riverbank along the scenic Yellowstone.

A copy of the investigators report was provided to The Associated Press by the office of Montana U.S. Sen. Max Baucus.

The investigators report chalks up the immediate cause of the spill to floodwaters that undermined the pipeline and left it exposed. As debris washing downriver piled up on the line, the pressure on the increased and it broke.

Investigators said that if Exxon s emergency procedures had called for the immediate closure of upstream valves, the crude oil release volume would have been much less and the location of the release would have been identified far more quickly.

The report also faulted Exxon for not having contingency plans in place to notify pipeline controllers that the Yellowstone was flooding.

Investigators, however, found little fault in the steps the company took in the lead-up to the spill.

Despite warnings from city officials in Laurel that the riverbank was eroding, Exxon continued to run crude beneath the Yellowstone after a survey found a section of pipeline leading away from the river was still buried more than 6 feet deep.

Exxon s field observations and their ... depth of cover survey took reasonable precautions to address the flooding of the Yellowstone River it the spring and early summer of 2011, the investigators wrote.

The investigation into the spill remains open. Whether any violations occurred remains under review, said spokesman Damon Hill with the Department of Transportation s Pipeline and Hazardous Materials Safety Administration.

Exxon Mobil spent $135 million on its response to the spill, including cleanup and repair work.

An Exxon spokeswoman said Wednesday the company still was analyzing the report.

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