The judge overseeing the federal prosecution of Luigi Mangione, the man accused of killing a health insurance executive in Manhattan, on Friday warned the U.S. attorney general to keep quiet about him to ensure a fair trial

As Mr. Mangione, 26, pleaded not guilty to a murder charge that could bring the death penalty, the judge, Margaret Garnett, made it clear that she wanted to depoliticize the circuslike atmosphere surrounding the case.

Mr. Mangione has become a hero to some Americans who say they are disgusted with the nation’s for-profit health system and an emblem of chaos for Attorney General Pam Bondi, who had announced that the government would seek capital punishment against him “as we carry out President Trump’s agenda to stop violent crime and Make America Safe Again.”

On Friday, scores of Mr. Mangione’s supporters came to the Thurgood Marshall United States Courthouse on Foley Square, with many waiting in line for hours outside. Trucks with his image circled around.

Inside the packed courtroom, Judge Garnett directed prosecutors to convey her caution against public commentary to the interim U.S. attorney, Jay Clayton, and asked that he pass on the message to Ms. Bondi.

Mr. Mangione’s lawyers had already complained about Ms. Bondi’s public statements. On April 1, even before he was indicted, Ms. Bondi said that the government would pursue the death penalty to carry out Mr. Trump’s agenda.