BILLINGS, Mont. (AP) -- Federal investigators say ExxonMobil's (NYSE:XOM) delayed response to a pipeline break beneath Montana's Yellowstone River made the spill far worse than it otherwise would have been.

Department of Transportation investigators say the volume of the 1,500-barrel spill could have been cut by about two-thirds if pipeline controllers in Houston closed off the line as soon as problems emerged.

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

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

The July 1, 2011, spill fouled 70 miles of riverbank along the scenic Yellowstone. Exxon spent $135 million on cleanup and repair work.

An Exxon spokeswoman declined comment Wednesday, saying the company was still analyzing the report.

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