The National Transportation Safety Board found Line 6B of the Lakehead pipeline system in Marshall, Mich., ruptured July 25, 2010. Authorities at pipeline operator Enbridge, however, didn't recognize the pipeline had ruptured until 17 hours later.
A report from Inside Climate News, backed up by NTSB records, found Enbridge operators in Canada believed they were receiving false alarms from the pipeline's system. Line 6B was undergoing a shut down in Indiana before the spill and operators believed pressure drops represented air pockets in the line, not a rupture, the reports found.
After the rupture, operators pumped oil under high pressure for one hour and later for 45 minutes before the break was confirmed.
The NTSB said the rupture on the pipeline measured about 6 feet by 4.5 inches at its widest location.
Enbridge early this month issued a proposal to the Michigan Public Service Commission to replace more than 200 miles of Line 6B, part of the oil pipeline system transporting so-called tar sands oil from Canada.