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The Tohono O'odham Nation, a Native American tribe in southern Arizona opposed to building a border wall that could imperil wildlife and artifacts in the area's fragile landscape, is accusing the Defense Department of failing to consult with it in line with federal requirements.

In a letter Feb. 7 to Defense Secretary Mark Esper, tribal Chairman Ned Norris Jr. said the department should have engaged in "meaningful consultation" with Tohono O'odham leaders because funding for the barrier is coming out of the agency's budget.

"The Nation respectfully requests that DOD immediately engage in government-to-government consultation ... and that no appropriated funds be expended on border barrier construction activity until such consultation has occurred," Norris wrote in the letter, which was shared with NBC News.

Last year, the Army Corps of Engineers said the Defense Department had awarded multimillion-dollar contracts to build a steel wall in Arizona, up to 30 feet tall in some sections. The Pentagon told Congress this month that it plans to divert $3.8 billion in military funding to pay for more wall construction along the U.S.-Mexico border. Democrats, including Sen. Kyrsten Sinema of Arizona, and some Republicans have opposed diverting the military's financial resources for the wall.

So far, the Trump administration has completed more than 100 miles of wall along the southern border, with more than 40 miles planned for southern Arizona along Organ Pipe Cactus National Monument and the adjacent Cabeza Prieta National Wildlife Refuge.

The Tohono O'odham Nation's reservation — the second largest in the U.S. by land holdings — is east of Organ Pipe, a 516-square-mile federally controlled park home to unique species of cactus recognized as a UNESCO ecological preserve.

The Tohono O'odham Nation says various sites at Organ Pipe carry historical significance, including one at Monument Hill, where crews over the past week began blasting the land and tearing down ancient saguaro cacti to make way for the wall. The area was once used for religious ceremonies by a distinct tribe known as the Hia-C'ed O'odham, and it is where bodies of Apache and other indigenous fighters were buried, according to the Tohono O'odham Nation's historic records.

The tribe says bone fragments have been found at Monument Hill, as well as near another site, Quitobaquito Springs, where construction crews working on the wall found remains believed to be human.

"They're disturbing sacred areas," Norris said in an interview. "It's disgraceful to see how much blasting is going on and how the remnants of our ancestors are being disturbed by that blasting."

Saturday, protesters, including O'odham activists, marched in opposition to the border wall construction.

Dozens of Indigenous families marched to the Ajo Border Patrol station yesterday to protest #BorderWall construction and the desecration of O’odham land. @CBPArizona are you listening? pic.twitter.com/a13BsZ5whW — Laiken Jordahl (@LaikenJordahl) February 16, 2020

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Norris said the government is supposed to have consulted with his tribe based on Executive Order 13175, which says agencies must have "an accountable process to ensure meaningful and timely input by tribal officials in the development of regulatory policies that have tribal implications." The policies can involve proposed actions on or off Indian lands.

"It's clear they're obligated to have government-to-government consultation with the tribe," said Norris, who became tribal chairman in June.

The Pentagon didn't immediately respond to a request for comment Monday about Norris' letter.

The Trump administration has been able to use federal waivers, including bypassing the Native American Graves Protection and Repatriation Act, to advance its border wall project.

The REAL ID Act, enacted in the wake of the terrorist attacks of Sept. 11, 2001, allows the government to work around certain laws in the name of national security.

In addition, a Supreme Court ruling in 1988 diminished Native Americans' ability to preserve sacred sites on federally controlled lands, such as national parks.

However, executive orders under the administrations of Presidents Bill Clinton and George W. Bush sought to accommodate tribal sovereignty and ensure that tribes are consulted, said David Martinez, an associate professor of American Indian studies at Arizona State University.

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But that still hasn't happened to full effect. Amid protests in 2016 over the building of the Dakota Access oil pipeline in North Dakota, citizens of the Standing Rock Sioux reservation complained that they weren't adequately consulted by the Army Corps of Engineers.

"For most Americans, they don't feel any connection to these places and people. Indian communities and our issues are just abstractions," Martinez said.

"Look at the reaction when Notre Dame burned down. You feel an emotional connection to that, even if you're not Catholic," he said. "That kind of emotional connection is abundant in the case of the border issues for the Tohono O'odham."

Tribal law expert Richard Monette, a law professor at the University of Wisconsin-Madison, said that it's rare for federal agencies to seek waivers in cases related to tribes and that the government simply doesn't consult with them as it should.

"Not many waivers can be couched in homeland security language," Monette said. "For better or worse, this wall can."

President Donald Trump has made building the wall a central promise of his immigration policy.

The southern border along Organ Pipe, west of Tucson, was once at the forefront of drug smuggling and illegal crossings, but the numbers of illegal crossings and apprehensions have fallen in recent years.