DETROIT (AP) — The Michigan Supreme Court heard arguments Wednesday that could wipe out charges against former Gov. Rick Snyder and eight others in the Flint water scandal, as lawyers challenged a rarely used, century-old method to investigate crimes and file indictments.

It’s the most significant attack by the defense in the 16 months since Snyder and others were indicted by Judge David Newblatt, who served as a one-person grand jury in Genesee County with evidence offered in private by the attorney general’s office.

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People charged with felonies in Michigan typically have a right to a hearing to argue there’s not enough evidence to send a case to trial. But the one-person grand jury process doesn’t allow it: If a judge decides to indict, the case moves directly to the trial court.

John Bursch, a lawyer for former state health director Nick Lyon, said the constitution’s separation of powers is violated when a member of the judiciary decides who gets charged.

“Because a judicial indictment is not only an oxymoron but a structural error, the only proper remedy is dismissal. … Judges are umpires who call balls and strikes,” Bursch said, quoting U.S. Chief Justice John Roberts. “They are not in the business of throwing the ball or swinging the bat, because to do so changes their role.”

Chief Justice Bridget McCormack seemed most troubled by the one-person grand jury, which was created in 1917 but has rarely been used in recent years, mostly only in Genesee and Wayne counties. She described Bursch’s arguments as “compelling.”

“If the judge is just investigating, I don’t hear Mr. Lyon to be complaining about that,” McCormack said. “It’s when the judge also gets to make a charging decision, which is an executive branch decision, that the constitutional problems arise, right?”

Nine people, including Snyder, are charged with various crimes tied to the disastrous decision in 2014 to switch Flint’s water supply to the Flint River without treating the water to reduce corrosion. As a result, lead contaminated the system for more than a year.

Snyder is facing misdemeanors, not felonies, but the indictment came from Newblatt.

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Justice Richard Bernstein wondered if the attorney general’s office in such high-profile cases should be doing everything to prevent any perception “that the government is cutting corners.”

Assistant Attorney General Chris Kessel said people charged with crimes don’t have a right to choose the process. He said the one-person grand jury was raised by Wayne County prosecutor Kym Worthy as the route to take.

The method compels witnesses to testify in secret or face consequences.

“This was handled correctly. … If the Legislature wants to create a limiting principle or they want to try to do away with the one-person grand jury, that’s an issue for the Legislature,” Kessel said.

Genesee County prosecutor David Leyton has used it in some violent crimes in the Flint area. He said it protects witnesses who fear retaliation if they had to testify in court at a probable cause hearing.

“We can at least get the information we need for charging purposes,” Leyton said.

Justice Elizabeth Clement will not participate because she worked as Snyder’s counsel before joining the court.

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