Story highlights The Canadian operator of an oil pipeline that cracked allegedly failed to make repairs

The firm apparently recognized structural problems years earlier before the oil spill

The 2010 spill was the most expensive on-shore oil spill in U.S. history, authorities said

Enbridge says the personnel involved "were trying to do the right thing"

The operator of an oil pipeline that cracked in 2010 and gushed nearly a million gallons of oil into a Michigan creek and river failed to make repairs and take appropriate action after recognizing structural problems several years earlier, the National Transportation Safety Board said on Tuesday.

The agency said Enbridge Inc. took more than 17 hours to shut down operations after the 30-inch-diameter pipeline ruptured and released crude oil into a wetland area near Marshall, Michigan, said NTSB Chairwoman Deborah A.P. Hersman. The NTSB, which often investigates plane and bus crashes, also monitors pipeline accidents.

According to the NTSB, the incident was the largest oil spill in the Midwestern United States. Federal officials say cleanup costs have exceeded $800 million.

The Pipeline and Hazardous Materials Safety Administration proposed a $3.7 million civil penalty, while recommending additional measures to improve training, oversight and accountability.

"Learning about Enbridge's poor handling of the rupture, you can't help but think of the Keystone Kops," Hersman said.

The pipeline, owned by Enbridge, split open and started spewing oil about 6 p.m. on July 25, 2010, according to the NTSB report. The 6½-foot long and 5-inch wide rupture gushed for 17 hours and 19 minutes.

Enbridge and the Environmental Protection Agency estimated the pipeline discharged more than 840,000 gallons of oil into Talmadge Creek, which feeds into the Kalamazoo River, which in turn flows from near the city of Battle Creek into Lake Michigan. The pipeline normally carries 190,000 barrels of oil per day from Griffith, Indiana, to Sarnia, Ontario.

During the spill, three shifts of employees monitoring the pipeline from the command center in Edmonton, Alberta, received multiple warning alarms, but dismissed them as being caused by column separation, or bubbles of vapor in the pipeline, according to the NTSB investigation.

"Instead of stopping the flow, Enbridge staff twice pumped more oil -- about 81% of the total release -- into the ruptured pipeline," Hersman said.

People in the area called 911 to report the smell of gas or oil, but the emergency responders didn't find the leak, and were unaware of the pipeline in the area, the report found.

Had crews known about the Enbridge pipeline, that knowledge could have prevented additional damage, investigators concluded.

The oil spill was eventually discovered by a local utility worker and reported to Enbridge's control center, where the flow was stopped.

The leak was caused by cracks forming on the pipe in an area where the waterproofing tape had pulled away and the pipe was corroding, investigators said.

The 41-year-old carbon steal pipe was inspected by an Enbridge contractor for rust in 2004 and cracks in 2005.

"The defect that caused the rupture at Marshal was misclassified," and as a result it "remained in the pipeline unabated until the rupture," wrote Ravindra Chhatre a member of the NTSB staff.

The inspections identified the problems in the area of the eventual leak, but engineers, following what the NTSB called "ambiguous" regulations, decided the problems weren't bad enough to be fixed.

The company's response to the leak was faulted by the NTSB, which said the initial work done by the company's employees was ineffective and the closest spill response contractor was 10 hours away.

The NTSB staff concluded that if the required oil spill-response plan had been thoroughly reviewed before the leak, it would not have been approved.

Enbridge Inc. Chief Executive Officer Patrick D. Daniel responded to the report on Tuesday, saying, "The experienced personnel involved in the decisions made at the time of the release were trying to do the right thing.

"As with most such incidents, a series of unfortunate events and circumstances resulted in an outcome no one wanted," he said.

"Safety has always been core to our operations," said President Stephen J. Wuori of Liquids Pipelines, Enbridge Inc. "Our intent from the beginning of this incident has been to learn from it so we can prevent it from happening again, and to also share what we have learned with other pipeline operators."