A U.S. government agency has approved investigators' findings that Canadian pipeline builder Enbridge Inc. knew about cracks that led to a 2010 leak of more than three million litres of oil into a Michigan river.

The most expensive onshore cleanup in U.S. history, which is nearly complete, has already cost about $800 million US.

A five-person panel of the National Transportation Safety Board (NTSB) voted Tuesday to accept the conclusions of investigators on the cause of the leak.

Matt Fox, an NTSB investigator, told board members on Tuesday that the Calgary-based company was aware of the cracks in its pipeline near Marshall, Mich., but failed to address them adequately before the rupture.

The pipeline started leaking on July 25, 2010, into Talmadge Creek near Marshall, about 96 kilometres east of Grand Rapids.

'Learning about Enbridge's poor handling of the rupture, you can't help but think of the Keystone Kops.'—NTSB chair Deborah Hersman

The oil spread into roughly 56 kilometres of the Kalamazoo River. The spill fouled wildlife habitat and resulted in the closure of a large swath of the river to boaters and anglers. All but a few hundred metres of the river have since reopened.

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

"Why didn't they recognize what was happening? What took so long?" she said in statement.

She said that despite alarms and pressure differentials, Enbridge staff twice pumped more oil, about 81 per cent of the total release, into the ruptured pipeline.

Hersman said that oil gushed from the rupture for more than 17 hours before the leak was discovered.

"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.

Enbridge officials said the spill has prompted the company to improve its operations and training.

"Safety has always been core to our operations," said Stephen Wuori, president of Enbridge's liquids pipelines division. "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."

U.S. regulators have proposed a record $3.7 million civil penalty against Enbridge.

Enbridge case watched closely in B.C. and Alberta

The results of the review are being closely watched in B.C. and Alberta, where an Enbridge proposal to build a $5.5-billion, 1,177-kilometre Northern Gateway pipeline from the oilsands to the West Coast is being opposed by environmentalists concerned about the danger of oil spills.

B.C Environment Minister Terry Lake said the report on the Enbridge spill is "pretty damning," but that doesn't mean the Northern Gateway project shouldn't go ahead. Lake said the B.C. government will take the report into account as it studies Enbridge's plan.

"When you read about the culture at Enbridge, that's worrisome," Lake said in an interview.

"But again, we need to fully understand this. This happened two years ago. What changes have occurred at that company to ensure that this kind of reaction doesn't occur in the future?… I view this as more information to help us formulate a position."

But B.C. NDP Leader Adrian Dix said the government of Liberal Premier Christy Clark should stop fence-sitting and join him in rejecting the Northern Gateway project.

"The long-term effects for the people of Michigan of what happened in the Kalamazoo River will be felt for a long time and have been serious in terms of health, in terms of the environment, in terms of the economy," Dix told reporters in Vancouver.

"Here in British Columbia, those risks to our economy, to our environment have to be part of the process, and they haven't been."

The federal and Alberta governments support the Northern Gateway project.

Bob Rae, the federal interim Liberal leader, said the NTSB report on the Enbridge spill "speaks to the need to make sure we have strong regulation and that the regulation has integrity." Rae was speaking in Fort McMurray, Alta., where he is touring the oilsands.

With files from The Associated Press and The Canadian Press