DETROIT – The Canadian operator of an oil pipeline that ruptured in southwestern Michigan two years ago, causing the most expensive onshore spill in U.S. history, failed to deal adequately with structural problems detected years ago and to respond appropriately to the catastrophe, the National Transportation Safety Board said Tuesday.

Enbridge Inc. knew in 2005 that its pipeline near Marshall, a city 95 miles west of Detroit, was cracked and corroded, but it didn't perform excavations that ultimately might have prevented the rupture, NTSB investigators told the five-member board in Washington before it approved the findings and 19 safety recommendations.

Enbridge didn't realize the pipeline was gushing oil into the Kalamazoo River and an enjoining creek for more than 17 hours, when a gas company worker pointed it out, and during that time Enbridge control center personnel twice pumped more oil into the ruptured line, investigators found.