“Federal statutes give (the Federal Energy Regulatory Commission) exclusive authority to govern various aspects of the interstate natural gas market,” she said.

ONEOK used to own Kansas Gas Service, but it split off its utilities into ONE Gas in 2013. KGS, Oklahoma Natural Gas and a Texas gas utility are part of ONE Gas.

Kansas Gas Service spokeswoman Dawn Ewing said neither KGS nor ONE Gas is involved in the lawsuit.

The school district and Learjet initially filed separate cases under state-level antitrust laws in 2005, but they were eventually combined into one.

In 2011, a district court ruled that a federal law pre-empted the state laws that the plaintiffs cited so the lawsuit couldn’t continue. The 9th Circuit Court of Appeals overruled last year, allowing the suit to proceed after plaintiffs appealed.

If the Supreme Court rules in favor of the defendants, no compensation will be given to the plaintiffs unless the Federal Energy Regulatory Commission penalizes ONEOK. If justices rule in favor the plaintiffs, the case would need to return to district court for trial. It’s unclear how much money could be at stake.