DETROIT — The Canadian operator of an oil pipeline that ruptured in southwestern Michigan two years ago, causing the most expensive onshore spill in US history, failed to deal adequately with structural problems detected years ago and did not 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 did not perform excavations that might have prevented the rupture, agency investigators told the five-member board in Washington before it approved the findings and 19 safety recommendations.

Enbridge did not realize the pipeline was gushing oil into the Kalamazoo River and a 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,’’ said the safety board chairwoman, Deborah Hersman, 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 and insufficient review of Enbridge’s oil spill response plan by the US Department of Transportation’s Pipeline and Hazardous Materials Safety Administration.

The safety board does not have the power to regulate pipeline companies, but its 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 .

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.