The federal government Monday announced that a controversial copper and gold mine in Alaska would cause substantial environment damage, could not receive a permit with its current proposed plan, and ordered the company to offset those impacts in order for the project to move forward. The U.S. Army Corps of Engineers sent a letter to Pebble Limited Partnership, the company behind the proposed mine, that said the footprint of the mine’s adverse impacts would cover nearly 3,000 acres of wetlands, 132.5 acres of open water and 130 miles of streams, along with areas affected by other infrastructure. Pebble has 90 days to reply with a plan to mitigate the damage. While the letter does not outright block the mine, environmentalists and former officials call it a significant setback for the project that has in recent weeks attracted opposition from prominent Republicans including the President’s son, Donald Trump Jr., Vice President Mike Pence’s former chief of staff Nick Ayers, and Fox News’ Tucker Carlson. The Army Corps said in a statement that the project “as currently proposed, cannot be permitted” under the Clean Water Act. The Army Corps cited a federal code that requires Pebble to restore, enhance or preserve an area comparable to that which would be damaged by the mine. That massive undertaking could potentially delay the project from getting its long-sought permit until after the US election in November, according to three current and former Environmental Protection Agency officials, two of whom did not want to be identified for fear of retaliation. Pebble CEO Tom Collier dismissed the significance of the letter, telling CNN it was a routine notice that didn’t impede the progress of the project. “All of this is an anthill made into a mountain,” Collier said, adding that the letter was not a surprise to company officials, that it was a normal part of the permitting and review process. “We’ve already got an additional plan. It’s a plan we’re confident of. We’re mapping it,” he said. Collier added that the effort has been ongoing for more than three weeks. Pebble has sought for more than a decade to develop a mine in Alaska’s Bristol Bay watershed, one of the world’s last pristine salmon spawning grounds and has encountered regulatory roadblocks in both the Obama and Trump administrations. The Bristol Bay region produces nearly half of the world’s annual wild sockeye salmon catch. In 2014, the Obama-era EPA all but shut the project down by invoking a provision of the Clean Water Act. As CNN previously reported, the EPA told staff scientists it was no longer opposing the mine after President Donald Trump met with Alaska Gov. Mike Dunleavy aboard Air Force One. Last month, it seemed the Pebble Mine got a major push forward when the Army Corps issued a final report concluding the Pebble Mine project would not cause long-term harm to one of the world’s largest remaining salmon runs. Former EPA Regional Administrator Dennis McLerran, who oversaw the team that studied the project under the Obama administration, said the letter is a major turnaround for the corps, which has largely appeared to support the project thus far. “The letter is truly extraordinary,” McLerran said. “It just demonstrates that there are major adverse impacts from this project that need to be mitigated. And in some cases…in my view, cannot be mitigated.” Collier said the new letter was “wholly unrelated” to recent criticism of the mine from Republicans. Trump Jr tweeted earlier this month that the Bristol Bay headwaters and surrounding fishery are “too unique and fragile to take any chances with.” Meanwhile, the company has launched an advertising effort to push the Trump administration to support the mine. “President Trump, continue to stand tall, and don’t let politics enter the Pebble mine review process,” a video Pebble posted on YouTube said. Democratic presidential nominee Joe Biden has said that if he’s elected his administration will stop the project. Conservationists and tribal representatives in Alaska agree that the Army Corps’ letter marks a significant hurdle for the mine, and some disagreed with Collier of Pebble that such stipulations are a normal part of a permitting process. “This letter represents a total and complete shift inside the Army Corps on Pebble Mine,” said Shoren Brown, director of the Alaska Heritage Campaign. “The Army Corps has finally figured out what the rest of us have known for a decade.” Chris Wood, president and CEO of Trout Unlimited, described the mitigation requirements laid out in the Army Corps’ letter as a “heavy lift.” “It’s a significant roadblock for the Pebble mine to overcome,” Wood said. Alannah Hurley, executive director of the United Tribes of Bristol Bay, argued that there’s no way for Pebble to fully offset the mine’s environmental damage. “It is impossible for Pebble to mitigate the devastation this mine will have on our Native cultures, our way of life that has been sustained for thousands of years by the pristine lands and waters of the Bristol Bay watershed,” Hurley said.