DETROIT (AP) — A Canadian company’s failure to deal adequately with cracks in an oil pipeline and its slow response to a 2010 rupture in southwestern Michigan likely caused the most expensive onshore oil spill in U.S. history, 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 at a meeting in Washington.

Investigators also faulted Enbridge control center personnel for twice pumping more oil into the line after the spill began and failing to discover what had happened for more than 17 hours, when an employee of a natural gas company notified them.

The board voted to approve the findings and 19 recommendations for safety improvements after testimony concluded.

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 River 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 NTSB Chairman Deborah 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. 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 improvements 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 U.S. Department of Transportation’s Pipeline and Hazardous Materials Safety Administration last week proposed a record $3.7 million civil penalty against Enbridge.

Oil began leaking from a 6.5-foot-long gash 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 laxity about following company procedures, investigator Barry Strauch said.

“Learning about Enbridge’s poor handling of the rupture, you can’t help but think of the Keystone Kops,” Hersman said. “Why didn’t they recognize what was happening? What took so long?”

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 rupture 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.

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?”