FLAGSTAFF, Ariz. (AP) – An Arizona man serving 90 years in prison on assault and firearms charges will have that sentence significantly reduced after a federal appeals court ruled that prosecutors did not prove he was a member of a federally recognized American Indian tribe, the first step to charging him with felony offenses that occurred on reservation land.

The split decision from the 9th Circuit U.S. Court of Appeals overturns Damien Zepeda's conviction on eight of nine charges, leaving a single count of federal conspiracy that carries a maximum sentence of 5 years. Zepeda is an enrolled member of the Gila River Indian Community, but the court said in a recent ruling that prosecutors did not prove beyond a reasonable doubt that his bloodline of one-quarter Pima and one-quarter Tohono O'odham derived from a tribe recognized by the U.S. Bureau of Indian Affairs.

The Tohono O'odham Nation of Arizona is on the BIA's list of federally recognized tribes, as is the Gila River Indian Community, which is made up of Pima and Maricopa Indians. But Zepeda's enrollment certificate didn't specify whether Tohono O'odham referred to the Arizona tribe or the collective population in Arizona and Mexico. The court said it would not make that determination on behalf of the jury.

A two-part test determines who is Indian for purposes of federal jurisdiction for crimes on reservation land, which result in far more stiff penalties than in tribal courts for the same offenses. The first requirement is that a defendant's blood line must derive from a federally recognized tribe.

The Arizona U.S. Attorney's Office said it was reviewing the decision but had indicated to Zepeda's attorney, Michele Moretti, that it would seek to have the case reviewed by a larger panel of the court, she said.

“I expect there to be a fight and push back,” Moretti said. “I think it's something that will be tested.”

A 2008 indictment against Zepeda charged him with conspiring with his brothers to assault his former girlfriend and two others on the Ak Chin reservation south of Phoenix. Authorities say they were armed with a handgun and a shotgun.

The appellate court has ordered that the district court resentence Zepeda on the federal conspiracy charge, which it said applies equally to everyone, everywhere within the United States.

Zepeda's brother had testified that their father was American Indian and their mother Mexican but Moretti said her client never held himself out to be American Indian. While the government could prove that his bloodline is linked to a federally recognized tribe through witness testimony, Moretti said she doesn't believe it can establish that he had cultural, economic or social ties to a tribe – the second part of the test.

Former federal prosecutors called the ruling bizarre and said it adds a level of complexity to already complex federal Indian law.

“It's a little like asking a jury whether the sun rises in the morning and sets at night,” said former Arizona U.S. Attorney Paul Charlton. “There's no debate about the fact that this individual is a Native American and subject to federal jurisdiction and trial. He won this case on a technical reading of the law and, as a result, will receive a greatly reduced sentence.”

Charlton and former Colorado U.S. Attorney Troy Eid, who are not involved in the case, said prosecutors would be prevented from retrying Zepeda because of double jeopardy.

Dissenting Judge Paul Watford said the court should have found as a matter of law that Zepeda's bloodline does indeed stem from a federally recognized tribe and remanded the case to the district court for a new trial, not reverse his convictions.

The majority said even if the court was permitted to make that determination, “which we are not,” it still would have been compelled to largely overturn Zepeda's conviction on grounds that the government did not prove it.

“As Zepeda's argument indicates, this is a factual inquiry and one that was not decided by the jury in this case nor could it have been as it was never presented to the jury,” Judge Richard Paez wrote for the majority.

Carole Goldberg, a law professor at the University of California-Los Angeles, said the ruling is part of a line of federal decisions that struggle with the definition of “Indian” for federal criminal jurisdiction and make addressing the cases more difficult for prosecutors. She said there already is widespread concern in Indian Country that federal prosecutors are too reluctant to pursue reservation crimes, and federally recognized tribes will be surprised to learn that their status as sovereign nations is being questioned.

The lack of clarity “suggests a need for rethinking the entire system of Indian Country criminal justice,” said Goldberg, co-editor and co-author of Cohen's Handbook of Federal Indian Law.