Three environmentalist groups have filed a federal lawsuit challenging the U.S. Army Corps of Engineers over its environmental analysis of the planned New England Clean Energy Connect hydroelectric corridor, which the groups oppose.

The Appalachian Mountain Club, Natural Resources Council of Maine and Sierra Club Maine allege in the lawsuit, filed Tuesday in U.S. District Court, that the Army Corps failed to rigorously assess the transmission corridor project’s environmental impact on western Maine in an analysis completed in July.

The Army Corps is one of the last groups that must sign off on the project, and it is expected to issue a positive determination for the developers, Central Maine Power owner Avangrid and Canadian utility Hydro-Quebec. The July report said that the Army Corps had determined the project would have “no significant impact” on the environment, but the agency has yet to issue a final ruling on the proposal.

In their complaint, the groups said the Army Corps should have studied and prepared an “environmental impact statement,” which is more rigorous and covers a broader range of issues than the “environmental assessment” that was performed on the proposed Maine project.

A spokesman for the Army Corps declined to comment on the lawsuit.

In its report, the Army Corps found that the 145-mile-long corridor, which would cut through about 53 miles of undeveloped forestland, would have no significant environmental impact. The lawsuit’s plaintiffs said they disagree with the finding and believe it was reached without adequate due diligence.

The July report was not released publicly, and the Natural Resources Council said it was able to obtain a copy only by filing a freedom of information request.

“The Army Corps of Engineers has abdicated its responsibility to assess the significant, harmful, and long-lasting impact that CMP’s massive transmission line would have on Maine’s North Woods,” Natural Resources Council Staff Scientist Nick Bennett said in a statement. “The Army Corps’ decision is a slap in the face to all Mainers. The evidence and testimony presented to the Army Corps made it clear that the CMP corridor is not in the public interest and is opposed by an overwhelming majority of Maine people.”

Clean Energy Matters, a political action committee funded by CMP and Avangrid to support the corridor project, dismissed the lawsuit as an attack on “the integrity of military service members and other public servants.”

“After two years, millions of dollars, and more than a dozen attempts to use legal action to derail the Clean Energy Corridor, (the Natural Resources Council) and its allies have yet to succeed,” Clean Energy Matters Executive Director Jon Breed said in a statement. “Their conduct is shameful.”

The lawsuit asks a federal court to set aside the Army Corps’ environmental assessment and the finding of “no significant impact” and declare that the decision to prepare an assessment instead of an environmental impact statement violates federal laws. Such a ruling would likely significantly delay the project because Avangrid needs approval from the Army Corps to move ahead with construction of the corridor.

Sierra Club Maine, one of the three environmental advocacy groups joining the lawsuit, said the Army Corps failed to assess the broader impacts of the transmission line by not producing a full environmental impact statement.

“The criteria for requiring an environmental impact statement have absolutely been met, and the Army Corps of Engineers is derelict in its duty not to have done so,” said Becky Bartovics, Sierra Club Maine’s volunteer leader.

Bartovics said the controversy over whether the project would have a net benefit in reducing greenhouse gas emissions, along with various other impacts to the environment and local communities, argue for requiring the full impact statement.

One of the project’s chief opponents, Sandi Howard, director of Say No to NECEC, released a statement following the announcement of the lawsuit, calling the lack of a full review dangerous. She noted that the agency had done full impact statement reviews of similar proposed transmission line projects in New Hampshire and Vermont.

“The Army Corps of Engineers decided to not conduct a full environmental impact statement for CMP’s corridor despite the obvious environmental risks of building the largest Maine project since the (Maine) Turnpike,” Howard said. “An (impact statement) is a more stringent environmental review than the environmental assessment the Army Corps finalized in July 2020.”

A full environmental impact statement provides a much broader appraisal of a project’s impact than an assessment, said Bennett, of the Natural Resources Council.

While an assessment might include a study of the project’s impact on the land corridor itself and maybe some adjacent land, Bennett said, a full study would look into a broader set of questions, including the societal impact of the project. And, he said, there would be a greater opportunity for public comments to be included in the study than there was in the assessment.

He said an environmental impact statement on the proposed Northern Pass, a similar project to bring electricity generated by hydropower in Quebec through New Hampshire to customers in Massachusetts, was 3,676 pages long and included 19 appendices. By contrast, Bennett said, the Army Corps’ assessment of the Maine project was just 163 pages long, and the report was not issued publicly for review and comment.

The Northern Pass project was eventually rejected by New Hampshire regulators.

Staff Writer Tux Turkel contributed to this story.