The NTSB found Enbridge knew of a defect in the pipeline five years before it burst .
The National Transportation Safety Board said at $800 million , it was the costliest onshore spill cleanup in U.S. history .
The National Transportation Safety Board found Enbridge knew of a defect in the pipeline five years before it burst .
The National Transportation Safety Board said at $800 million , it was the costliest onshore spill cleanup in U.S. history .