In northeast Minnesota, two new copper-nickel mining projects on the fringe of the state's Iron Range are inching closer to reality.

Both projects — one by the PolyMet mining company and the other by Twin Metals Minnesota — are controversial. They would usher in a new kind of mining to the state, promising new economic growth, but also new environmental risks.

But the two proposed mines are viewed differently by many current and former state and federal officials. That difference was underscored again this week, with a new group registering their opposition to the proposed Twin Metals mine.

More than two dozen former U.S. Forest Service staffers — including the recent head of the Superior National Forest — sent a letter to Trump administration officials Wednesday detailing their concerns and what they say is a new sense of urgency about the proposed copper-nickel mine on the edge of the Boundary Waters Canoe Area Wilderness.

It's just the latest volley over the prospect of copper-nickel mining in the Superior National Forest.

Twin Metals, a subsidiary of Chilean mining conglomerate Antofagasta, is vying to build an underground copper-nickel mine and processing facility along the shores of Birch Lake and the South Kawishiwi River, which lie in the Rainy River watershed and flow into the Boundary Waters about 5 miles to the northeast.

It's the project's location that the former Forest Service employees highlighted in their letter.

"Within the Rainy River watershed, there is simply no way to contain contamination without sacrificing the wilderness and the long-term ecologic and economic sustainability that it supports," wrote the 33 signatories, many of whom worked in the Superior National Forest in northeast Minnesota.

Their letter was addressed to U.S. Agriculture Secretary Sonny Perdue, who oversees the Forest Service, and Interior Secretary David Bernhardt. It was also sent to Minnesota Gov. Tim Walz.

"Irrefutable scientific studies and all of our experience tell us that in this extremely valuable, water-rich and highly interconnected place you simply cannot have both copper mining and healthy forests, water and communities," the letter concluded.

For its part, Twin Metals is already working with the U.S. Forest Service, the Minnesota Department of Natural Resources and other government regulators as it prepares a mine plan of operations, a spokesperson said, something it intends to file later this year.

That would kick off a multi-year environmental review and public process.

"Speculation about the impact of a potential mine at this time, before a proposal has even been submitted, is just that — speculation," the company said in a statement.

"For more than a decade, we have been conducting the largest geological study in Minnesota history, with more than $450 million already invested in scientific, geological and hydrological research."

The Trump administration has breathed new life into the Twin Metals project, after a series of decisions in the waning days of the Obama administration threatened to kill the mine proposal.

In late 2016, the Forest Service and the Bureau of Land Management refused to extend two federal mineral leases, which are critical to the Twin Metals mine plan.

At the same time, the Forest Service proposed a 20-year mineral withdrawal — essentially, a moratorium on new mining activity — on more than 200,000 acres of Superior National Forest land in the Boundary Waters watershed. The agency then launched a 24-month environmental review of that proposal.

But the Trump administration reversed those decisions, first by withdrawing the Forest Service's application for a mineral withdrawal and ending the environmental review of the proposal after 15 months, after the agency determined it hadn't revealed any new scientific information.

Then, the Bureau of Land Management proposed renewing Twin Metals' mineral leases for an additional 10 years, and took public comment on a draft environmental assessment of the plan.

Once the agency issues a final assessment, Twin Metals will be free to submit its mine plan.

The letter submitted to federal officials this week asks for the federal government to pause that process by completing the study of the proposed 20-year mining ban, and to suspend Twin Metals' leases until legal challenges to those lease reinstatements are heard in court.

The letter does not mention the proposed PolyMet mine, which earlier this year received the final permits it needs to begin construction. The company is now trying to raise the roughly $1 billion it will need to build the state's first ever copper-nickel mine.

Brenda Halter, who retired as Forest Supervisor for the Superior National Forest in 2016, helped draft the letter. She said all the signatories have professional experience working on projects related to the Boundary Waters.

Under her leadership, the Forest Service approved a land exchange in which it agreed to trade federal land where PolyMet intends to dig its open pit mine with privately held land elsewhere in the forest. That decision has since been challenged in federal court.

Halter acknowledged the role she played with PolyMet, which she said was to make sure the legal process and the science were followed to help inform the agency's decision, can seem contradictory with the stance she's taken on Twin Metals.

"[But] I feel very strongly on a personal and professional level that a copper-nickel mine adjacent to millions of acres of water-based ecosystem is simply an incompatible use," she said.