Questions about whether two federal agencies each conducted deep enough reviews of Central Maine Power’s controversial power line project in western Maine, and whether those agencies should have worked more closely together, dominated testimony Tuesday in a complex case before the First Circuit Court of Appeals in Boston.

At issue is whether the appeals court should reconsider the rejection in December of an injunction request in U.S. District Court aimed at blocking construction on a segment of the New England Clean Energy Connect project.

During more than an hour of questioning, a panel made up of Judge David Barron and Judge Ojetta Rogeriee Thompson sought to understand the specific impacts of the project and the degree to which the venture triggered various levels of federal oversight. They also wondered about a lack of coordination between permit reviews done by the U.S. Army Corps of Engineers, the lead defendant in the case, and the U.S. Department of Energy, which also issued a permit for the project.

The legal arguments presented Tuesday represent the latest skirmish in an ongoing battle between environmental activists and developers of the 145-mile transmission line.

Sierra Club Maine, the Natural Resources Council of Maine and the Appalachian Mountain Club sued the Army Corps last October in hopes of blocking the corridor. They alleged the Corps conducted a flawed and inadequate environmental review for the power line project, and they asked a federal judge to issue a preliminary injunction that would have prevented construction while they fought in court.

But U.S. District Judge Lance Walker denied that request in December. Since then, project developers have begun clearing land and setting poles for the $1 billion project along an existing transmission corridor owned by CMP.

But they are prohibited from working in a disputed, 53-mile stretch of largely undeveloped forest from the Quebec border to The Forks in Somerset County. The plaintiffs say clearing that corridor would fragment the largest contiguous temperate forest in North America and have dire impacts on wetlands and wildlife.

They also asked Walker to consider granting an injunction while the higher court reviews their appeal. He denied that motion.

After Walker’s rejections, the plaintiffs asked the federal appeals court to reconsider that decision. They argued in part that Walker took too narrow an approach in his ruling and did not consider all the potential harms from the project.

One of those approvals is a so-called presidential permit, which allows the power line to cross the Canadian border. That permit was issued by the U.S. Department of Energy in January, in the waning days of the Trump administration.

On Monday, the conservation groups went to court to add the federal energy department to their lawsuit. They allege that the agency also conducted a substandard review, and in doing so violated federal environmental laws. That request is pending.

U.S. Rep. Jared Golden, D-Maine, also has asked the Biden administration to rescind the permit and conduct a more rigorous review.

The overarching complaint by the groups is that the two federal agencies didn’t prepare full environmental impact statements that assess a range of impacts, which they say are required by law and would have included public comments. The agencies have differing authorities. The Army Corps oversees bodies of water, and the judges sought to understand the scope of affected waterways and wetlands – power line crossings versus filling or dredging, for instance – and the degree of federal oversight required.

The lawyer representing the groups, Kevin Cassidy, said that while the waterbodies were spread out, the cumulative impact of the project should have triggered a fuller review by the Army Corps. He also said both agencies should have looked at issues such as forest fragmentation, which is a key impact of creating the corridor.

But Jeffrey Hall, the lawyer representing the Army Corps, said just because the impacts are dispersed doesn’t mean the entire project needs a fuller federal review. And Joshua Dunlap, the lawyer representing CMP, noted that Maine regulators already had reviewed the wetland and stream impacts in detail, and still issued state permits.

“The state is the primary regulator here,” Dunlap said.

The issue of review carried over to the analysis done by the Department of Energy, which issued a less-complete review known as an environmental assessment in approving the presidential permit. Both judges tried to determine whether the energy department should have done a fuller environmental impact statement, and why the Army Corps didn’t take the energy department’s subsequent judgments into account.

“How is it possible there’s no agency communication?” Thompson asked. “This sounds crazy.”

Dunlap suggested that both agencies found the project wouldn’t cause significant impacts, which is a legal standard for issuing permits.

The judges now will take the matter under consideration. There is no set time for issuing a decision.

The NECEC project would transmit 1,200 megawatts of hydroelectricity from Canadian utility Hydro-Quebec to the New England grid, and would be funded by Massachusetts ratepayers.