WASHINGTON (Reuters) - U.S. Supreme Court justices appeared cautious on Tuesday about potentially broad consequences in a death penalty case in which they are being asked to declare huge swathes of the state of Oklahoma as officially part of a Native American reservation.

FILE PHOTO: The Supreme Court is seen ahead of the start of it's new term in Washington, U.S., October 1, 2018. REUTERS/Aaron P. Bernstein/File Photo

Patrick Murphy, sentenced to death for the 1999 roadside mutilation slaying of his girlfriend’s former lover, has asked the high court to throw out his murder conviction because the crime occurred in a location, Henryetta, that should be viewed as reservation land outside the reach of Oklahoma criminal law.

The justices heard an hour of arguments in Oklahoma’s appeal of a lower court ruling that sided with Murphy, a member of the Miscogee (Creek) Nation, a Native American tribe based in the state.

The state and President Donald Trump’s administration both argued that the Creek Nation’s formal reservation was eliminated by Congress around the time Oklahoma achieved U.S. statehood in 1907. Murphy’s lawyers disagreed. The justices weighed a complex historical record that started with the forced relocation of Native Americans, including the Creek Nation, to Oklahoma in a traumatic 19th century event known as the “trail of tears.”

If the justices side with Murphy, he could be given a new trial in federal court without facing the death penalty. His arguments would mean that the Creek Nation’s jurisdiction covers 3 million acres (12,140 square km) in eastern Oklahoma, including the city of Tulsa, according to the state.

The justices focused their concerns on the practical impact of a ruling that would give the Creek Nation considerably more power than it currently wields. If lands viewed as being part of the Creek Nation were considered a reservation, tribe members would not, for example, have to pay state taxes.

The state also said that potentially thousands of prison inmates could challenge their sentences in the same way as Murphy if he were to win.

The tribe itself has argued that its reservation was never actually terminated, noting that it has a police force, operates hospitals and paves roads. But, as several justices pointed out, the tribal government has only limited authority and in the past century has not aggressively pressed the same arguments raised by Murphy’s lawyers.

“The historical practice for a century has been against you,” conservative Justice Brett Kavanaugh told the tribe’s lawyer, Riyaz Kanji.

“Stability is a critical value in judicial decision-making, and we would be departing from that and creating a great deal of turmoil,” Kavanaugh added.

Fellow conservative Justice Samuel Alito asked: “How can it be that none of this was recognized by anybody or asserted by the Creek Nation, as far as I’m aware, for 100 years?”

The tribe received some backing from liberal justices including Elena Kagan and Sonia Sotomayor. But fellow liberals Stephen Breyer and Ruth Bader Ginsburg seemed to share some of their conservative colleagues’ concerns.

The state appealed after the Denver-based 10th U.S. Circuit Court of Appeals ruled for Murphy last year.

Conservative Justice Neil Gorsuch, formerly a judge on the 10th Circuit, did not participate in the case, raising the possibility of a 4-4 split along ideological lines that would leave the lower court ruling favoring Murphy in place but not set any nationwide legal precedent.

A ruling is due by the end of June.