Contractors hired to help build President Trump’s border wall are destroying part of a national monument that’s sacred to a Native American tribe in Arizona.

U.S. Customs and Border Patrol contractors said work has started to demolish a mountain that's part of the Organ Pipe Cactus National Monument, which borders Southern Arizona and the Mexican state of Sonora. The demolition is a preliminary step in erecting a 30-foot wall of steel meant to deter illegal crossing into the U.S.

“The construction contractor has begun controlled blasting, in preparation for new border wall system construction, within the Roosevelt Reservation at Monument Mountain in the U.S. Border Patrol’s Tucson Sector,” U.S. Customs announced in a statement Thursday. “The controlled blasting is targeted and will continue intermittently for the rest of the month.”

The monument is part of the Roosevelt Reservation, a 60-foot-wide strip of federal land that runs along the border states of California, New Mexico and Arizona, but it's got special environmental and cultural significance.

In 1976, the United Nations Educational, Scientific and Cultural Organization designated the land an International Biosphere Reserve in order to conserve its unique resources. The National Parks Service calls the Organ Pipe “a thriving community of plants and animals.” And the land holds cultural significance to the Tohono O’odham nation, as it once served as the homeland for the native people.

“Human stories echo throughout this desert preserve, chronicling thousands of years of desert living,” the National Parks Service website details.

An environmental monitor will be present as the demolition takes place, according to the U.S. Border Patrol. But local officials are skeptical.

“DHS has consistently failed in its legal obligation to consult with the tribes, and this is only the latest example,” U.S. Rep. Raul Grijalva told the Arizona Daily Star in a statement. “DHS has consistently failed in its legal obligation to consult with the tribes, and this is only the latest example.”

The monitor’s presence also didn’t prevent the contractors from ignoring strict public-land designations that should have prevented them from uprooting cactus and drilling into an aquifer nested deep within the land.

The National Parks Service warned the Trump administration in September that building the wall would have environmental and cultural impacts on the land and the Tohono O’odham people. “I equate the impact of building a wall to building a wall over your parents’ graves or your family graveyard,” Tohono O’odham Nation Tribal Chairman Ned Norris Jr. said at the time.

This isn’t the first time the Trump administration has trounced on the sacred lands of America’s native people. In 2017, Trump ended a monthslong struggle between the U.S. government and the Sioux people in North Dakota over the installation of the Dakota Access Pipeline on the Standing Rock Indian Reserve, when he issued an executive order streamlining the project’s approval process.