U.S. Supreme Court justices, in a case on Tuesday testing the scope of Native American tribal police powers, leaned toward backing the authority of a tribal police officer in Montana to stop and search a non-Native American motorist on a public road on reservation land.

The justices heard arguments in a case focusing on drug-related charges brought against a man named Joshua James Cooley, who is not a Native American, after Crow tribal police officer James Saylor in 2016 found methamphetamine and firearms in his vehicle, which was parked on a roadside on reservation land, Reuters reports.

Cooley argued that tribal police lacked jurisdiction over him as a non-Native American even though his vehicle was searched on a public road situated on reservation land. Lower courts ruled in favor of Cooley. The Supreme Court heard the federal government’s appeal backing tribal authority.

Some justices expressed concern about public safety if tribal officers lacked power to stop and detain non-Native Americans. Conservative Justice Clarence Thomas wondered what would happen if an officer encountered a known serial killer.

Chief Justice John Roberts, another conservative, noted that under Supreme Court precedent, tribes do retain some inherent authority to oversee their affairs.

“What could threaten that more than the idea that you can’t do anything about somebody within the reservation that you have good reason to believe is violating criminal law?” Roberts asked.