Rep. Betty McCollum used part of a congressional budget hearing Tuesday to grill U.S. Interior Secretary David Bernhardt over decisions to advance copper-nickel mining in northeastern Minnesota.

McCollum, chairwoman of a powerful House subcommittee that controls Interior’s budget, asked Bernhardt to address the Trump administration’s “reckless push to mine and drill in our public lands, including in my home state of Minnesota, where sulfide ore mining poses a critical risk to the Boundary Waters Canoe Wilderness Area and Voyageurs National Park.”

The Minnesota Democrat also asked Bernhardt why his office has not provided documents she requested related to key decisions on the copper-nickel mine proposed by Twin Metals Minnesota and its owner, the Chilean mining firm Antofagasta.

In recent weeks, McCollum asked Interior officials for documents on their decision to reverse an Obama administration decision to terminate Twin Metals’ federal mining leases, and to explain why they decided to cancel a two-year U.S. Forest Service study that would have examined how copper-nickel mining on Superior National Forest lands might affect the nearby Boundary Waters wilderness.

Bernhardt told McCollum his agency is working on providing the documents.

“There’s no way that we’re going to approve something that is destructive to the Boundary Waters, but there are processes we go through to analyze that,” Bernhardt testified. “We can’t approve a mine plan of operation that would cause jeopardy or adversely modify or destroy critical habitat. All of those things will be looked at, at the appropriate time.”

The exchange came just days after a group of nearly three dozen retired U.S. Forest Service employees from Minnesota wrote a letter to the Trump administration expressing similar concerns about the two key Twin Metals decisions.

Writing to Bernhardt and U.S. Department of Agriculture Secretary Sonny Perdue, the group expressed “grave concerns” about the proposed underground copper mine.

“It is indisputably a high-risk business and there is no mine plan or design feature that eliminates the risk,” the letter said.

Twin Metals is one of two heavily contested copper mines proposed for Minnesota’s Iron Range. The other, by PolyMet Mining, has cleared most of Minnesota’s state regulatory hurdles. The Twin Metals plan, which is still in the early stages of regulatory approvals, has drawn even greater concern from conservationists because it would lie in the same watershed as the Boundary Waters Canoe Area Wilderness (BWCA), meaning that any pollution released from the site would run downstream into the pristine wilderness refuge.

The letter, dated May 1, calls on the federal agencies to stick to the Obama-era decision to terminate Twin Metals’ federal mining leases in order to protect the BWCA, and to restart the canceled two-year Forest Service study that would have examined how copper-nickel mining on Superior National Forest lands might affect the nearby BWCA.

“While our concerns are based on science and local expertise, simple common sense tells us that with upward of 75% of all known copper deposits still available for development, there are surely more ecologically-suited places to meet demand for copper than directly upstream from millions of acres of water-rich wilderness,” the letter said.

More than 30 former Forest Service employees signed the letter. They include rangers, wildlife biologists and soil scientists, and said that together they have nearly 1,000 years of forestry experience.

In an interview, Brenda Halter, a retired Forest Service hydrologist now living in Ashland, Wis., said she helped organize the effort. The group isn’t affiliated with any organization, she said, and the letter was a grassroots effort among Forest Service employees who worked in Minnesota.

“We just reached out to people that we knew,” Halter said. “In this watershed, if there’s a leak or failure there is so little opportunity to capture it or remediate it without essentially trading off the wilderness. You can’t fix it.”

The letter notes how frequently copper mines experience some type of failure resulting in pollution. It also mentioned concerns about damage from acid mine drainage, the sulfuric acid that is generated when sulfide minerals in ore and rock are exposed to water and air during mining.

Recent failures of mine tailings dams, the structures that permanently store mine waste, have heightened concerns globally. World Mine Tailings Failures, a group tracking accidents, has recorded 42 failures at tailings dams around the world in the past decade. Following the disastrous collapse in January of a tailings dam at the Córrego do Feijão iron ore mine in Brazil that killed at least 200 people, the International Council of Mining & Metals, a leading industry group, has launched an independent review of standards for managing tailings dams.

David Ulrich, a spokesman for Twin Metals Minnesota, said the company has not yet submitted its official mine plan to regulators, and that the Forest Service retirees are only speculating about impacts. The company has been studying the proposed mine for more than a decade, he said, and invested $450 million “in geological and hydrological research to ensure potential future mining will be environmentally safe.”

The company’s mine plan, to be submitted within a few months, will trigger extensive reviews by numerous regulatory agencies, he said.

“We expect to meet and, where possible, exceed all environmental standards put in place by regulators,” Ulrich said. “If we can’t, the project will not move forward.

When asked for a response to the letter, an Interior spokeswoman pointed to Bernhardt’s exchange with McCollum Tuesday.