When they filed charges against Michigan officials last year, including former Gov. Rick Snyder, prosecutors said they had failed to protect the safety and health of Flint residents who had contracted high levels of lead and of water in the city. jumped into the Flint River in April 2014. However, prosecutors appointed by Attorney General Dana Nessel, a Democrat, relied on a single-member jury to indict Mr. Snyder, a Republican, and eight others, including the former state health director and former chief medical officer. . The Supreme Court said Tuesday that single-person courts could not be used in this way. Three defendants, not including Mr. Snyder, had challenged the use of the one-member jury, but the court ruling also seemed likely to upset the prosecution of the other defendants. An official from Ms. Nessel’s office said prosecutors were reviewing the decision. It was not clear if they intended to bring new charges. At least nine people died from Legionnaires in the Flint area from June 2014 to October 2015. The water crisis, which has led to rising lead levels among thousands of people in Flint, has left countless families distrusting the water supply, even as city officials insist that it is now safe for consumption. The allegations made by Ms. Nessel’s team were a second attempt to prosecute officials for what happened in Flint. Before Ms. Nessel took office, her Republican predecessor helped oversee cases against 15 state and local officials for crimes as serious as manslaughter. But Ms. Nessel’s team dismissed those cases in 2019 before filing new charges against several of the same officials.