SIOUX FALLS, S.D. (AP) — Nine Native American tribes in South Dakota, North Dakota and Nebraska are suing the federal government in a bid to stop exploratory drilling for graphite near a sacred site in the Black Hills.

A small group of demonstrators has been protesting at the drilling location and at the mining company’s headquarters since they learned ground was broken on the drilling project in late April.

The tribes filed their federal lawsuit Thursday in South Dakota against the U.S. Forest Service and U.S. Department of Agriculture, alleging that the agencies violated federal law by greenlighting a project near a site called Pe’Sla, a meadow in the central Black Hills used for tribal ceremonies, prayer and youth camps year-round.

The project is the latest point of tension between tribes and mining interests in the lush pine-covered Black Hills, which encompass over 1.2 million acres (485,000 hectares), rising from the Great Plains in southwest South Dakota and extending into Wyoming.

The region is a yearly destination for millions of tourists boasting such attractions as Mount Rushmore and wildlife-filled state parks. Yet for even longer, it has been sacred to Sioux tribes who call the area He Sapa and consider it “the heart of everything that is,” according to the complaint.

Some of the landscape has already been altered by the gold rush of the 1870s that developed the region and displaced Native Americans. And in recent years, a new crop of miners driven by rising gold prices have sought to return to the landscape.

The complaint said the project by Rapid City-based mining company Pete Lien & Sons would impact the use of Pe’Sla for traditional, cultural and religious purposes by the tribes, and that the Forest Service did not consult with the tribes before approving the project.

Parts of Pe’Sla are owned by Sioux tribes after they bought the land in 2012, 2015 and 2018, and an agreement between the tribes and the Forest Service established a two-mile (three kilometer) buffer zone on public lands around the site, according to the complaint.

Because Pe’Sla was not included as an affected area and no environmental review was conducted, the approval violates the National Historic Preservation Act and National Environmental Policy Act, the lawsuit alleges.

Pete Lien & Sons, which supplies materials like limestone, sand and gravel, did not return email requests or voicemails for comment Thursday and Sunday.

Frank Star Comes Out, president of the Oglala Sioux Tribe, said in a statement that the lawsuit is “a historic demonstration of unity” between the nine tribes. The tribes are separate, distinct federally recognized tribes sharing cultural and linguistic roots, but each with its own government and land base.

“We as Lakota people have been coming and praying and holding ceremony at these places for over 2,000 years,” said Wizipan Garriott, president of Indigenous advocacy group NDN Collective and a member of the Rosebud Sioux Tribe. “And so us being here is a continuation of countless generations before us. And it’s important that these sacred places be protected for future generations to come.”

The project was granted a permit from the Forest Service in February without conducting an environmental review because the agency said it met the requirements for a categorical exclusion, like having a duration of less than a year and not posing impacts to environmental and cultural sites.

But tribal opponents disagree that those requirements were met and said drilling projects are often a first step leading to future mines.

Besides the lawsuit from the tribes, NDN Collective and other environmental groups filed a request for a temporary restraining order and preliminary injunction to halt the project.

Some of the drilling pads are in the buffer zone around the site, according to NDN Collective. The project calls for the company to drill up to 18 holes down some 1,000 feet (300 meters) into the Earth to collect samples.

On Thursday, opponents demonstrated with signs reading “Protect Pe’Sla” and “Sacred ground not mining bound” near two drilling pads to block access. NDN Collective said the Forest Service told them drilling was paused for the rest of the day and the contractors were sent home.

The Forest Service said in a statement that it had no comment on the project when asked for a response.

“The Forest Service does not comment on the specifics of the case or on issues that are part of ongoing legal proceedings,” the statement said.

It is unclear when drilling began, but NDN Collective said it noticed drilling pads in operation last week. The group said protest actions will continue as needed to protect Pe’Sla.

“As Lakota, we pray as long as we need to,” Garriott said.