TULSA, Okla.—A year and a half after the Supreme Court ruled that much of eastern Oklahoma remains a Native American reservation, law-enforcement officials are settling into a changed legal world, learning new rules about who can be prosecuted on expanded tribal lands home to nearly two million people.

Now they face the prospect that those rules could change again.

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TULSA, Okla.—A year and a half after the Supreme Court ruled that much of eastern Oklahoma remains a Native American reservation, law-enforcement officials are settling into a changed legal world, learning new rules about who can be prosecuted on expanded tribal lands home to nearly two million people.

Now they face the prospect that those rules could change again.

Native Americans saw the July 2020 ruling in McGirt v. Oklahoma as a landmark victory in their long bid to reclaim tribal sovereignty after a history of forced removals and broken treaties. But it also reordered Oklahoma’s justice system, as suddenly hundreds of cases involving Native Americans could no longer be brought by state prosecutors or heard by state courts. That saddled federal and tribal courts with a deluge of cases they weren’t equipped for, sowing uncertainty for defendants and victims alike.

Oklahoma Gov. Kevin Stitt, saying the decision imperiled convictions and left crimes unpunished, petitioned the Supreme Court to have it overturned. The court rejected that request in January but agreed to hear a case that could partially restore the state’s power to prosecute crimes in Indian country. Its ruling in that case, Oklahoma v. Castro-Huerta, could potentially shake up the legal system again, this time with consequences that could span beyond Oklahoma. Arguments will be in April and a decision is expected before July.

“It feels like it would be whiplash,” Steve Kunzweiler, the Tulsa County district attorney, said of the prospect of another change. He saw many of the cases his office prosecuted transferred to federal court in the weeks after the Supreme Court’s 2020 decision—and could soon have to take some of them back.

Oklahoma Gov. Kevin Stitt has said the McGirt decision imperiled convictions and left crimes unpunished.

Photo: Stephen Pingry/Tulsa World/Associated Press

Within a week of the McGirt ruling, Mr. Kunzweiler had to tell the mother of two children who died of heat exhaustion in their father’s locked truck that he could no longer bring charges against the man. The reason: The victims were Cherokee, a detail he said he wouldn’t have known or considered days earlier.

“The look on her face was a classic 1,000-yard stare,” he said. “It was just a look of loss and despair, and there was nothing I could do about it.”

Understaffed for the new workload, federal prosecutors in 2020 picked up the case and hundreds of other homicides, rapes and assaults normally handled by local authorities. The FBI’s Oklahoma City field office became one of the agency’s busiest, borrowing scores of out-of-town agents versed in terrorism, counterintelligence and civil-rights investigations to work more like police detectives investigating shootings and beatings.

Federal judges poured in from other districts to help process the exploding docket of cases. After Julia O’Connell, Oklahoma’s federal public defender, put out a call for volunteers, her staff more than doubled, with attorneys working on folding card tables in cramped offices.

By the end of 2021, the U.S. attorney’s office in Tulsa said it had opened more than 1,000 cases involving Native Americans and routed another 2,000 to tribal courts, which generally handle lower-level offenses like burglaries and theft.

Clint Johnson, U.S. attorney for the Northern District of Oklahoma, in his office in Tulsa.

The Tulsa office’s initial call for volunteers became known in other U.S. attorney’s offices as the “McGirt tax.” Twenty-four prosecutors—from Pittsburgh, Seattle, Buffalo, N.Y., and elsewhere—were later hired to take on what Clint Johnson,

U.S. attorney for the Northern District of Oklahoma, called “a crushing onslaught of cases initially.”

One of them was Cymetra Williams, who cut her teeth as a federal prosecutor in New Jersey investigating healthcare fraud. In Tulsa she handles domestic-violence and sex-assault cases with Native American victims. While before she followed trails of tax records, she now peruses photographic evidence of abuse and other graphic clues.

“If there’s a fraud case, the bank is the victim usually,” Ms. Williams said. “You don’t have someone that hugs you after a detention hearing.”

State prosecutors like Julie Childress, who specialized in domestic-violence cases as an assistant district attorney in North Carolina, also stepped up. In her new federal role, Ms. Childress prepared and comforted a Cherokee woman ahead of a hearing that led to a white man being sentenced to nearly six years in prison for assaulting her.

“For those victims who have never even been to court, stepping inside a federal courtroom can be overwhelming,” she said.

Cymetra Williams, one of the many prosecutors hired to handle the onslaught of cases related to the McGirt decision.

Before the McGirt ruling, the U.S. attorney’s office in Tulsa had been bringing about 200 indictments a year. Last year, it handled more than 500. In the more rural Eastern District of Oklahoma, where the five main tribes are headquartered, violent-crime case filings rose from two in fiscal year 2019 to 299 in 2021. The office had just eight prosecutors before McGirt and now has 40, said

Doug Horn, senior litigation counsel for the Eastern District. Many of them are handling child-abuse cases, which now account for 40 % of the office’s workload.

McGirt’s impact on Oklahoma’s criminal justice overall isn’t easily characterized. Some offenders have ended up with a similar sentence to what they might have received in state court; others got harsher punishments, and some got off more easily.

Dozens of violent crimes have gone unpunished because federal statutes of limitation expired, witnesses died or evidence vanished. In one case, a man suspected of raping five women in the 1990s was charged by the state in 2020 after DNA evidence surfaced, but had his case dismissed on McGirt grounds. A five-year federal statute of limitations kept the Justice Department from adopting the case, allowing him to go free without facing trial. That issue, Mr. Horn said, “is a hole in the system we could not address.”

Critics of the McGirt decision, including Mr. Stitt, point to such cases, saying it is unfair to victims to watch cases get dropped or go unpunished.

The Illinois River outside of Tahlequah, Okla., the capital of the Cherokee Nation.

Tribal leaders reject the charge that justice is being left undone. “What the tribes want is the ability to make their own laws and be governed by them, and that means the ability to enforce those laws,” said Sara Hill, attorney general for the Cherokee Nation, which handled roughly 60 times as many criminal cases in 2021 than in an average year before McGirt.

On Tuesdays, known as “McGirt Days,” hundreds of people line the halls of the Cherokee Nation courthouse waiting for their cases to be heard. The Muscogee (Creek) and Cherokee Nations say they have spent millions of dollars expanding their justice systems to accommodate the workload.

For many of Oklahoma’s Native Americans, any upheaval is a small price to pay for a high-court ruling that reckoned with the government’s sometimes shameful treatment of the tribes.

The case was brought by Jimcy McGirt, a Seminole citizen who was originally convicted in state court and received two 500-year sentences and life without parole for sexually abusing a 4-year-old girl on Muscogee (Creek) Nation land. He argued that only federal prosecutors were entitled to try him because Congress had never formally extinguished the tribe’s sovereignty when Oklahoma became a state in 1907.

Prosecutor Julie Childress drives between her office and the federal courthouse in Tulsa.

In a 5-to-4 decision, the Supreme Court agreed. Justice Neil Gorsuch, writing for the majority, noted that the lands at issue, the terminus of the Trail of Tears, had been promised to the tribes in exchange for territory in Georgia and Alabama taken from them by white settlers.

The ruling, which led to the affirmation of five other reservations, didn’t much alter Mr. McGirt’s fate. He was retried in federal court and sentenced to life.

In the Supreme Court’s new case, the justices are considering whether to allow Oklahoma to retain jurisdiction over crimes committed by all nontribal citizens in the state, even if they happen on tribal lands. The case of Richard Ray Roth shows why that matters.

Mr. Roth, 42 years old, was convicted in state court of manslaughter in 2014 and sentenced to 20 years in state prison after killing a 12-year-old boy while driving drunk. Citing the McGirt decision, Mr. Roth, who isn’t a tribal citizen, successfully challenged his conviction on the grounds that the state of Oklahoma no longer had jurisdiction over his case because the boy was Cherokee and the crime happened on the Creek reservation.

Mr. Roth remains in prison while the Supreme Court decides in the case. If the justices rule against Oklahoma, Mr. Roth could be released as a result of a federal five-year statute of limitations on manslaughter.

That prospect is just one legacy of a ruling Oklahoma’s Republican governor still rejects. “With one Supreme Court ruling, they have torn our state apart and thrown up this jurisdictional chaos upon us,” Mr. Stitt said.

In court papers on the coming Castro-Huerta case, Oklahoma argues many cases in which non-Native-American convictions were overturned on McGirt appeals resulted in federal plea agreements that imposed lesser sentences than state courts had.

“As federal judges, prosecutors, and investigative agents scramble to keep up with caseloads many times larger than ever experienced or expected, the ability of the State of Oklahoma to prosecute non-Native-Americans in Indian country is vital to maintaining public safety,” the state wrote in its filings.

Legal experts said the outcome of that Supreme Court case could open battles in places where the jurisdictional balance has long been settled. Five other states—Texas, Nebraska, Louisiana, Kansas and Virginia—have filed court papers supporting Oklahoma.

“It adds an additional layer of complication and jeopardy for defendants,” said University of Oklahoma law professor Joseph Thai.

Mr. Thai is representing a man who was appealing his life sentence in state court for abusing his 5-week-old daughter when it was vacated on McGirt grounds because the girl was Native American. Federal prosecutors then took the case. But if the Supreme Court says the state also has jurisdiction, Mr. Thai said, his client could see his state court conviction reinstated while trying to fight federal charges.

“We are potentially looking at two barrels of a shotgun instead of one,” he said.

Justice Department officials said they were still assessing what a ruling in favor of the state in Castro-Huerta could mean for them and the dozens of prosecutors now detailed to Oklahoma. Meanwhile Mr. Kunzweiler, the district attorney, is bracing for the return of some cases to his office, now smaller because many left to work for the federal government or tribes.

“That chaos that existed at the beginning and that disruption—I think it is reasonable to anticipate that again,” he said.

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Write to Sadie Gurman at sadie.gurman@wsj.com