Minnesota’s Ojibwe Indian tribes say state regulators failed to do a complete cultural study and thus botched their environmental review of Enbridge’s proposed new oil pipeline across northern Minnesota.

In a regulatory filing this week, five bands asked the Minnesota Public Utilities Commission (PUC) to reconsider its recent decision on the environmental review and order that a “full historic properties review” be done.

The tribes and environmental groups have harshly criticized the Environmental Impact Statement (EIS) done by the Minnesota Department of Commerce on Enbridge’s proposed new Line 3, which would replace an aging and corroding pipeline. In December, the PUC rejected the EIS, but on narrow environmental concerns.

The EIS must be approved before the PUC’s final decision on Line 3, which had been expected in April but now may not occur until June.

Representatives of the Fond du Lac Band of Chippewa and the Mille Lacs Band of Ojibwe told the PUC last month that the EIS should have been rejected out of hand because it failed to include a formal tribal cultural resource study. Environmental reviews often account for such cultural resources as burial grounds and historic villages.

“The state’s historic properties work on the Line 3 Replacement project to date has been so inadequate that it could be used as a ‘what not to do’ example in future guidance,” said the filing by the Mille Lacs, Fond du Lac, White Earth, Leech Lake and Red Lake Indian bands. The approach “violates a host of state laws.”

In a statement Thursday, the Commerce Department said that it “has been, and is, committed to providing any and all information deemed necessary by the PUC to make this important decision on behalf of the state.”

A Commerce Department official in December told the PUC that a full tribal cultural survey should be done, but it’s not the department’s job. Instead, that study should be led by the U.S. Army Corps of Engineers in consultation with the tribes.

In December, the PUC also voted that construction can’t start on a new Line 3 — if it’s approved at all — until a tribal cultural survey is completed. The commission also said it would weigh any information from the cultural study that it could obtain before its final vote on Line 3. The study might not be done by that time, however.

The tribes, in their filing this week, said the PUC “effectively excused the [Commerce Department’s] failure” to do a full historic properties survey. “The commission has decided that it will make routing and need decisions on one of the state’s largest-ever construction projects without ever considering a full record on the impacts of historic properties,” the tribes said.

The $2.6 billion, 340-mile new Line 3 would transport Canadian oil to Enbridge’s terminal in Superior, Wis. New Line 3 would follow the same route as the existing Line 3 and five other Enbridge pipelines to Clearbrook, Minn. But from there, it would jog south to Park Rapids before running east to Superior.

Opponents of a new route say the pipeline would expose a new region of Minnesota — including pristine lakes and wild rice waters — to degradation from oil spills. Enbridge said it chose the new route because its existing pipeline corridor is congested and the Leech Lake Indian band has rejected building a new Line 3 through its reservation.

Enbridge’s pipeline corridor, including existing Line 3, runs through the Leech Lake and Fond du Lac reservations. The company’s proposed path for new Line 3 does not cross any reservations, though it runs through land where tribes have treaty rights to hunt, fish and gather.