The U.S. Supreme Court ruled Thursday that a large portion of eastern Oklahoma remains American Indian Reservation land.

Oklahoma’s jurisdictional landscape dramatically changed this morning when the U.S. Supreme Court ruled much of the state’s eastern half remains a Native reservation, thereby throwing past criminal convictions into question.

Justice Neil Gorsuch wrote in the court’s 5-4 decision that it was holding the federal government to its word, and that lands promised in treaties remain Indian reservation land for the purposes of federal criminal law.

In McGirt v. Oklahoma, Jimcy McGirt, a Muscogee (Creek) Nation citizen convicted of sex crimes against a child, argued the state should not be allowed to prosecute him, because the crime was committed within the tribe’s historical boundaries, and that the reservation was never disestablished. As a result, McGirt argued, he could only be tried in federal court.

This was the second case that cast doubt on Oklahoma’s criminal jurisdiction. In June last year, the Supreme Court said it would rehear arguments in the Sharp v. Murphy case. However, the high court appeared deadlocked at 4-4 when Gorsuch recused himself, because he had served on the 10th Circuit Court of Appeals when it ruled on the case. Patrick Murphy, who was charged with murder and sentenced to death, also argued that only the federal government could prosecute him. Federal law would also prevent him from receiving the death penalty.

In a joint statement issued Thursday, Oklahoma’s Five Civilized Tribes – composed of the Creek, Cherokee, Chickasaw, Choctaw, and Seminole Nations – said they plan to present Congress and the U.S. Department of Justice with an agreement addressing jurisdictional issues raised by the ruling.

“The Nations and the State are committed to ensuring that Jimmy McGirt, Patrick Murphy, and all other offenders face justice for the crimes for which they are accused,” the Fives Tribes said. “We have a shared commitment to maintaining public safety and long-term economic prosperity for the Nations and Oklahoma.”

Some in the legal community have questioned how criminal proceedings will be handled going forward, and area officials have suggested the change in jurisdictional power could place a burden on federal authorities. Oklahoma Solicitor General Mithun Mansinghani has reportedly argued in the past that the land in question was never part of a reservation, and that a ruling in favor of McGirt could mean the release of thousands of state prisoners.

District Attorney Jack Thorp, representing Cherokee, Wagoner, Adair and Sequoyah counties, said he respects the ruling, but he’s concerned, since all of District 27 is now considered Native American territory.

“The ruling affects my office in a large way. This now returns a lot of cases to square one, and that’s makes me concerned for those victims,” Thorp said.

Thorp added that cases wherein a Native American was convicted of one of the major crimes – murder, manslaughter, rape, assault with intent to kill, arson, burglary, and larceny – will have the opportunity to be retried in federal court.

Cherokee Nation Principal Chief Chuck Hoskin Jr. was adamant that crimes committed on reservation lands will face swift justice, and said he agreed with the high court’s decision.

“I agree with Justice Gorsuch’s opinion today that the U.S. government should be held to its treaty obligations, and its word,” he said. “The Cherokee Nation is glad the U.S. Supreme Court has finally resolved this case and rendered a decision which recognizes that the reservation of the Creek Nation – and by extension the reservations of the Cherokee Nation, Chickasaw Nation, Choctaw Nation, and Seminole Nations – were never diminished and that our respective governments were never dissolved.”

Depending on the circumstances, a crime committed in Indian Country could be sent to federal, tribal, or state courts. According to Thorp, if a defendant is Native and the victim is Native, federal authorities would have jurisdiction over felony cases and tribes over misdemeanors. The same situation applies if the defendant is Native and the victim is non-Native. If a defendant is non-Native and the victim Native, federal courts would have jurisdiction over both felonies and misdemeanors. The state would have jurisdiction over cases wherein both the defendant and victim are non-Native.

In Gorsuch’s ruling, he wrote that Oklahoma and its tribes have proved they are capable of working together. State and tribal officials have echoed the sentiments.

“The Nations and the State are committed to implementing a framework of shared jurisdiction that will preserve sovereign interests and rights to self-government while affirming jurisdictional understandings, procedures, laws, and regulations that support public safety, our economy, and private property rights,” said the Five Tribes. “We will continue our work, confident that we can accomplish more together than any of us could alone.”

The U.S. attorneys for Oklahoma’s Northern, Eastern and Western Districts – Trent Shores, Brian Kuester and Timothy Downing, respectively – have also suggested confidence in intergovernmental cooperation.

“As Oklahoma’s U.S. Attorneys, we are confident tribal, state, local, and federal law enforcement will work together to continue providing exceptional public safety under this new ruling by the U.S. Supreme Court,” the U.S. Attorneys said.

Oklahoma’s congressional delegation – including District 2 Congressman Markwayne Mullin, whose jurisdiction covers much of the affected area – said it is reviewing the decision and is ready to work with tribal and state officials to ensure criminals are brought to justice.

“Indeed, no criminal is ever exempt or immune from facing justice, and we remain committed to working together to both affirm tribal sovereignty and ensure safety and justice for all Oklahomans,” said the delegation.

Oklahoma Gov. Kevin Stitt also has been made aware of the ruling. He said his legal team has been closely following the case, and is reviewing the decision carefully.

“They will advise our team on the case’s impact and what action, if any, is needed from our office,” said Stitt.

Keri Thornton, Tahlequah Daily Press, and Janelle Stecklein, CNHI state reporter, contributed to this story.