Line 6B of the Lakehead oil pipeline system ruptured in 2010, spilling about 20,000 barrels of so-called tar sands oil into the Kalamazoo River in Marshall, Mich.
Deborah Hersman, chairwoman for the National Transportation Safety Board, said during a Tuesday hearing Enbridge knew of the defect in the line in 2005.
"In 2005, Enbridge detected the very defect that led to this failure," she was quoted by the Detroit Free Press as saying. "Yet for five years, they did nothing to address the corrosion or cracking at the rupture site and the problem festered."
The NTSB said the rupture on the pipeline measured about 6 feet by 4.5 inches at its widest location. Federal investigators said Enbridge overruled an analysis in 2005 that would've required excavation of the pipeline.
A preliminary investigation into the incident by the Pipeline and Hazardous Materials Safety Administration suggested Enbridge operators misinterpreted alarms that indicated a leak occurred in Marshall.
Hersman, the Press reports, added Enbridge was able to exploit "weak" PHMSA regulations.
Enbridge, in a statement, said it takes full responsibility for the incident, adding NTSB findings "are generally consistent" with an internal probe.