Michigan oil spill

The National Transportation Safety Board has approved the findings of its investigators that Canadian company Enbridge Inc.'s neglect of pipeline cracks and its slow response likely caused the most expensive onshore oil spill in U.S. history.

The board on Tuesday approved its staff's findings about the cause of a 2010 rupture near Marshall in southwestern Michigan that dumped about 843,000 gallons of heavy crude into the Kalamazoo River and an tributary creek.

Investigators say Enbridge knew five years before the rupture that cracks were forming in the pipe but didn't perform excavations that might have prevented it.

They also say Enbridge control center personnel twice pumped more oil into the line after the spill began and didn't discover what had happened for more than 17 hours.