Even when he was just a wanted man smiling in a surveillance picture, Luigi Mangione elicited a fervent response from some Americans. Now identified and charged in the brazen fatal shooting of a health care executive, his influence has persisted, even from behind bars.

Supporters, some of whom have championed his anti-insurance-industry message, have deluged him with correspondence in the federal Metropolitan Detention Center in Brooklyn. They have sent Mr. Mangione, 26, gifts and at least $500,000 for his defense fund. His lawyers created a website complete with a personal statement from the man himself and instructions on how to contact him.

“I am overwhelmed by — and grateful for — everyone who has written me to share their stories and express their support,” Mr. Mangione’s message said, adding, “mail has flooded M.D.C. from across the country, and around the globe.”

The positive response has horrified many Americans who were shocked by the brutality of the crime of which Mr. Mangione is accused: assassinating Brian Thompson, the chief executive of UnitedHealthcare, in Manhattan. But in the nearly three months since the shooting, the groundswell of interest and support has been sustained.

A rally was organized outside the Lower Manhattan courthouse where a hearing was to be held in his case Friday afternoon, with fliers trumpeting support “for people harmed & killed by insurance industry greed.”

In a 15th-floor hallway, about 100 young women lined benches and sat on the floor. Some wore red sweaters with white-collared shirts, an apparent homage to Mr. Mangione’s outfit during his last court appearance.

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