ST. PAUL — Minnesota officials need to do more work on the environmental review of Enbridge Energy’s proposed replacement for its aging Line 3 crude oil pipeline, which has drawn strong opposition from environmental and tribal groups.

The Public Utilities Commission voted 4-1 Thursday to declare that the environmental impact statement is “inadequate” and sent it back for revisions to the Minnesota Department of Commerce, which conducted the review, the Star Tribune reported. The PUC isn’t scheduled to decide the bigger question of whether the project is needed until late April at the earliest.

The PUC told the agency to revise three small parts of the EIS within 60 days, partly to include more information on how one particular alternative route could be altered to avoid environmentally sensitive geologic formations.

The PUC’s other questions also involved alternative routes. The main document runs about 2,000 pages and has about 12,000 pages of appendices.

Line 3 carries Canadian crude oil to Enbridge’s terminal in Superior, Wisconsin. It starts in Alberta and clips a corner of North Dakota before crossing Minnesota. Enbridge wants to follow the existing route to Clearbrook, Minnesota, then take a more southerly route to Superior. Enbridge’s preferred route would avoid two American Indian reservations crossed by the existing pipeline where tribal approval is seen as unlikely, but it would cross the pristine Mississippi River headwaters region and areas where Ojibwe bands still have treaty rights and harvest wild rice.

At Thursday’s hearing, representatives of the Fond du Lac and Mille Lacs bands testified that the EIS was inadequate because it lacked a formal tribal cultural resource study. The survey is underway, they said, but it won’t be finished until sometime next year.

“Once again, Native American interests have slipped through the cracks,” said Paul Blackburn, an attorney for Honor the Earth, an Indian-led environmental organization.

Calgary, Alberta-based Enbridge says the project is necessary because its existing Line 3, which was built in the 1960s, can run at only half its original capacity because it’s increasingly subject to corrosion and cracking, and its maintenance needs are expected to grow rapidly. The company says replacing it with a state-of-the-art pipeline would better protect the environment against oil spills, and would restore its capacity to 760,000 barrels per day, ensuring reliable deliveries of oil to Midwest refineries.

Enbridge estimates the cost of the overall project at $7.5 billion, including $2.6 million for the Minnesota segment. Construction is already underway in Canada and Wisconsin. Several protesters have been arrested in recent months at the pipeline site near Superior, where demonstrators have included veterans of the protests against the Dakota Access pipeline on the Standing Rock Reservation in North Dakota.