The Canadian operator of an oil pipeline that ruptured in southwestern Michigan two years ago, causing the most expensive onshore spill in U.S. history, failed to deal adequately with structural problems detected years ago and to respond appropriately to the catastrophe, the National Transportation Safety Board said Tuesday.

Enbridge Inc. knew in 2005 that its pipeline near Marshall, a city 95 miles west of Detroit, was cracked and corroded, but it didn't perform excavations that ultimately might have prevented the rupture, NTSB investigators told the five-member board in Washington before it approved the findings and 19 safety recommendations.

Enbridge didn't realize the pipeline was gushing oil into the Kalamazoo River and an enjoining creek for more than 17 hours, when a gas company worker pointed it out, and during that time Enbridge control center personnel twice pumped more oil into the ruptured line, investigators found.

"Learning about Enbridge's poor handling of the rupture, you can't help but think of the Keystone Kops," NTSB Chairman Deborah Hersman said at Tuesday's meeting. "Why didn't they recognize what was happening? What took so long?"

The report also faulted the government, citing weak regulation insufficient review of Enbridge's oil spill response plan by the U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration.

The NTSB doesn't have the power to regulate pipeline companies, but its safety recommendations carry significant weight with lawmakers, federal and state regulators, and industry officials. Results of its investigations sometimes are used in lawsuits.

The spill dumped about 843,000 gallons of heavy crude into the Kalamazoo and a tributary creek, fouling more than 35 miles of waterways and wetlands. About 320 people reported symptoms from crude oil exposure.

Enbridge's cleanup costs have exceeded $800 million, which Hersman said was more than five times greater than the next-costliest onshore spill — a 2005 release of 991,788 gallons by Chevron Pipeline Co. in Buras, La. That cleanup cost $150 million.

"This accident was the result of multiple mistakes and missteps by Enbridge," Hersman said. "But there is also regulatory culpability. Delegating too much authority to the regulated to assess their own system risks and correct them is tantamount to the fox guarding the henhouse. Regulators need regulations and practices with teeth — and the resources to enable them to take corrective action before a spill, not just after."

Enbridge officials said the company had improved its operations and training after the spill and would study the NTSB report to determine whether further steps were needed.

"Safety has always been core to our operations. 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," said Stephen J. Wuori, president for liquids pipelines.

The federal pipeline agency last week proposed a record $3.7 million civil penalty against Enbridge.

Oil began leaking from the 30-inch line, which runs from Griffith, Ind., to Sarnia, Ontario, around 6 p.m. on July 25, 2010. Even though alarms sounded repeatedly at the Enbridge control center in Edmonton, Alberta, staffers on hand misinterpreted them. Their failure to act reflected a "culture of deviance" about following company procedures, investigator Barry Strauch said.

Controllers ordered two restarts before the leak was discovered, sending 683,000 gallons — 81 percent of all the spilled oil — into the stricken line.

Patrick Daniel, Enbridge's CEO, said: "We believe that 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."

Investigators traced the 6.5-foot-long gash to moisture seeping beneath plastic tape on the steel pipe, causing corrosion. Instruments detected a series of corrosion and fatigue cracks in 2005, investigator Matt Fox said. Eventually, those cracks linked together to form the rupture.

Enbridge failed to grasp the significance of the cracks or to put together findings from separate tests of pipe wall thickness in a way that would have led them to dig up the pipe for a visual inspection, he said.

The report urged the pipeline agency to strengthen its rules for inspecting cracked lines and to develop training requirements for control center staff. It also called for public awareness campaigns in areas with underground pipelines and better planning to deal with pipeline emergencies.

Hersman said the Michigan spill raised some of the same issues as the 2010 Pacific Gas & Electric Co. pipeline explosion that killed eight people and injured 58 others in San Bruno, Calif.

"While our findings raise red flags about the safety of these two companies, they should also force us to ask hard questions of this vital industry," she said. "With more than 2.5 million miles of pipeline running through this country — enough to circle the Earth 100 times — we have to ask, 'Are these companies representative of others?' If the answer is yes, we can expect to be back here again discussing the same issues with a different company."

Deb Miller, 57, who lives near the Ceresco dam on the Kalamazoo River, watched parts of the NTSB meeting online.

"We know and realize now that our river is never going to be the same," Miller said. "What can we learn from this and share with the rest of the country so nobody has to go through what we went through?"


Flesher reported from Traverse City, Mich. AP reporter Joan Lowy in Washington contributed to this story.