Exxon Mobil Corp.'s delay in responding to a major pipeline break in Montana's Yellowstone River in July 2011 worsened the impact of the 1,500-barrel spill by about two-thirds, the federal government said Wednesday.

Pipeline controllers in Houston failed to isolate the rupture as soon as problems emerged the Associated Press reports, instead allowing crude oil to drain from the 12-inch pipeline for another 46 minutes.

A report by the Department of Transportation states that Exxon's faulty emergency procedures did not call for the immediate closure of upstream valves and the company did not have contingency plans in place to notify pipeline controllers that the river was flooding prior to the break.

Landowners along the river have sued Exxon saying it did not respond adequately to the spill and didn't do enough to prevent it.