In a letter Monday to the Government Accountability Office (GAO), the congressmen said the spills raise “serious questions” about pipeline owner TC Energy’s management of the line, and whether the Pipeline and Hazardous Materials Safety Administration is providing adequate oversight.

“The public has a legitimate expectation that the Keystone Pipeline System managed by TC Energy operate safely and without repeated incidents that damage the environment and threaten the public’s health and security,” according to the letter signed by House Energy and Commerce Chairman Rep. Frank Pallone, of New Jersey; Transportation and Infrastructure Chairman Peter DeFazio, of Oregon; Railroads, Pipelines and Hazardous Materials subcommittee Chairman Dan Lipinski, of Illinois; and Energy subcommittee Chairman Bobby Rush of Illinois.

GAO spokeswoman Sarah Kaczmarek said Tuesday that the nonpartisan watchdog agency received the congressional request and is evaluating it.

“At this point, no decisions have been made,” she said, adding that the process could “take some weeks.”

Calgary-based TC Energy, formerly known as TransCanada, reported the spill in North Dakota on Oct. 29. The pipeline returned to service on Nov. 10 after approval of a repair and restart plan by the federal pipeline safety agency.

The company did not immediately respond to an Associated Press phone message seeking comment on the House Democrats’ request for action.

The cause of the leak has not been disclosed and cleanup work continues near the small town of Edinburg, North Dakota, state regulators said.

The House Democratic leaders said in their letter that the North Dakota leak follows spills in South Dakota in 2016 and 2017.

“Yet we are faced with the third occurrence of a significant pipeline leak that has devastating impacts to both the environment and nearby communities,” they wrote

Crude began flowing through the US$5.2 billion pipeline in 2011. It’s designed to carry oil across Saskatchewan and Manitoba, and through North Dakota, South Dakota, Nebraska, Kansas and Missouri on the way to refineries in Patoka, Illinois and Cushing, Oklahoma. It can handle about 87 million litres (23 million gallons) daily.

It is part of a system that also is to include the proposed US$8 billion Keystone XL pipeline designed to transport oil from Hardisty, Alta. to terminals on the Gulf Coast in Texas.

The proposed Keystone XL pipeline has drawn opposition from people who fear it will cause environmental damage, and the House Democrats in their letter to the GAO called the proposed expansion “particularly concerning.”