Exxon Mobil has upped the amount of oil that spilled last summer into the Yellowstone River near Billings Montana.

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Exxon Mobil officials now say 1,509 barrels of oil spilled into the Yellowstone River during a pipeline break in Montana last summer.  That's an increase of more than 500 barrels over the company earlier estimates. Of that amount only about 10 barrels were ever recovered.

Company Spokesman Alan Jeffers says they recalculated the volume after discovering the pipeline was completely severed during the July 1 2011 accident near Laurel.

Jeffers says pipeline breaches typically involve a crack or fissure. That was the assumption used to craft the initial estimate.

The company has estimated costs related to the spill to be around $135 million.

More than 1,000 Exxon Mobil contractors were involved in the cleanup effort that recovered less than 1 percent of the total spill.