Exxon Mobil says 1,509 barrels of oil spilled into the Yellowstone River during a pipeline break in Montana last summer — an increase of more than 500 barrels over the company's earlier estimates.

Spokesman Alan Jeffers said Thursday the company recalculated the volume after discovering the pipeline was completely severed during the July 1 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 of $135 million.

More than 1,000 Exxon Mobil contractors were involved in the cleanup effort. Only about 10 barrels of crude were recovered — less than 1 percent of the total spilled.