The cleanup has been difficult because Line 6B was carrying diluted bitumen, or dilbit, an unconventional form of oil derived from Canada's oil sands that has defied traditional oil recovery methods. The long-debated Keystone XL pipeline, which would run from Alberta to the U.S. Gulf Coast if the entire project is approved, would also carry dilbit. The most controversial portion, in Nebraska, would cross one of the largest and most important freshwater aquifers in the nation, the Ogallala. TransCanada, the builder of the Keystone XL, plans to use the same type of leak detection system as Enbridge used on Line 6B, according to TransCanada's website.

In the Enbridge control room, the first six high-priority alarms sounded within five minutes of the initial rupture on line 6B. Two hours later at 8 p.m., the controllers on duty left work at the end of their normal 12-hour shift. According to the documents, they didn't inform their replacements of the six alarms from Marshall, and by the time the new team arrived the alarm indicators had cleared and disappeared from the computer screens in the control room.

At 4 a.m., the controllers on the new shift in Edmonton restarted the pipeline to end what they thought was a planned shutdown. Within minutes, three alarms rang in succession at 4:12, 4:16, and 4:22. Pumps were pushing oil into the 30-inch line with the force of a fire hose. Between 4:36 and 4:57, three more high-priority alarms sounded. At about 5 a.m. line 6B was shut down again. The Enbridge team decided to call it a "false alarm."

The same team tried a second restart at 7:10 a.m. High priority alarms continued to sound at 7:12, 7:35, 7:37, and 7:42. The team called for more pressure but there wasn't enough power available, so the line was shut down at about 8 a.m.

According to the documents, the next shift took over without clear knowledge of the earlier events. Operators were preparing for a third restart when the emergency phone rang in the Edmonton control room. A local gas utility worker in Marshall was on the line with news of the spill. At that point Enbridge initiated its emergency response procedures.

InsideClimate News reporters Elizabeth McGowan and Lisa Song contributed to this report.