As the U.S. government prepares to execute Lezmond Mitchell, the only Native American man on federal death row, the leaders of the Navajo Nation have asked President Trump to reduce Mitchell’s sentence to life imprisonment.

“We strongly hold to our cultural, traditional, and religious beliefs that life is sacred,” Navajo Nation President Jonathan Nez and Vice President Myron Lizer wrote in a recent letter.

Mitchell is scheduled to be executed on Aug. 26. The federal government recently resumed executions after a 17-year hiatus. (Capital punishment is used more frequently by the states than by the federal government.)

But putting Mitchell to death would violate Navajo beliefs, Nez and Lizer say. And they add that his execution would challenge tribal sovereignty.

Mitchell and a co-defendant, both of whom are Navajo, killed a Navajo woman and her 9-year-old granddaughter on a Navajo reservation in 2001. Federal prosecutors typically can’t seek the death penalty for a major crime committed on a reservation without the tribe’s approval, and most tribes reject capital punishment.

The Navajo Nation did not agree with pursuing the death penalty for Mitchell’s murder charges. So federal prosecutors also charged him with carjacking, a lesser offense, and they were able to seek the death penalty on that charge over the tribe’s objections.

The Mitchell case would be the first time in the modern history of the death penalty that the federal government executed a Native American for a crime committed entirely on tribal lands and against fellow tribe members.

The Navajo leaders say that since 2002, the tribe has repeatedly informed the U.S. government “of the Navajo Nation’s opposition to the death penalty in Mr. Mitchell’s case citing Navajo cultural teachings that stress the sanctity of life and instruct against the taking of human life for vengeance.”

They note that the daughter and mother of the victims also opposes the death sentence in this case.

Pursuing the death penalty anyway has “marginalized” the sovereignty of the Navajo Nation, Nez and Lizer say.

Mitchell has appealed his conviction, arguing racial bias may have tainted his jury trial. His appeal was rejected this spring by the Ninth Circuit Court of Appeals.

However, even as they allowed his execution to move forward, two judges wrote opinions expressing deep concerns about the federal government’s actions in Mitchell’s case. Judge Morgan Christen called the pursuit of the death penalty in the case “a betrayal of a promise made to the Navajo Nation.”

She agreed that the government was acting within the bounds of the law.

“But that the government had the right to make this decision does not necessarily make it right,” wrote Christen’s colleague, Judge Andrew Hurwitz. He suggested President Trump consider using his authority to call off the execution.

The Navajo Nation sent its plea to the president a week and a half ago; a spokesman tells NPR the tribe has not yet received a response.