Federal prosecutors say the man accused of starting one of the most destructive fires in California history was fascinated by fire and by Luigi Mangione, who became a populist hero to some after he was charged with murdering a health insurance executive.

Jonathan Rinderknecht, 30, is accused of intentionally setting a fire in the Santa Monica Mountains that later exploded into the Palisades fire, which killed 12 people and destroyed thousands of homes across the Pacific Palisades neighborhood of Los Angeles in January last year.

A trial memorandum released by federal prosecutors last week paints a portrait of Mr. Rinderknecht in the weeks before the fire as a lonely and erratic man who was angry at the world, particularly the rich.

Federal prosecutors say that in December 2024, the month before the fire, Mr. Rinderknecht had been living alone in an apartment in North Hollywood and working as an Uber driver.

He seemed to have followed the case of Mr. Mangione, who is charged with fatally shooting Brian Thompson, the chief executive of UnitedHealthcare, in New York City on Dec. 4, 2024.

In the wake of the shooting, Mr. Mangione became a folk hero to some people, who saw him as striking a blow against the health insurance industry and its profits. (Mr. Mangione is set to stand trial on a second-degree murder charge in September.)

Mr. Rinderknecht searched for Mangione-related news, using the search terms “free Luigi Mangione,” “lets take down all the billionaires” and “reddit lets kill all the billionaires,” according to court documents.

When investigators later asked Mr. Rinderknecht why someone might commit arson in the Palisades, he said it would be out of resentment of the rich, and he compared such a fire to the murder for which Mr. Mangione was charged. “We’re basically being enslaved by them,” he told investigators.

Mr. Rinderknecht was arrested in October and has been charged with three arson-related counts. He has pleaded not guilty, and his lawyers maintain that the fire was sparked by fireworks. Mr. Rinderknecht is being held at the Metropolitan Detention Center in downtown Los Angeles. His trial is set for June.

His lawyer, Steven Haney, pushed back on federal prosecutors’ latest narrative. He said that they were politicizing the case and offering “wild motives and conspiracy theories.” He said that the focus on Mr. Mangione revealed a lack of concrete evidence.

“If fascination with Luigi Mangione is evidence of arson, the U.S. attorney’s office is going to need a much bigger courtroom — because they’ll have to indict half the country,” he said in an email.

Prosecutors pointed to other factors that might have affected Mr. Rinderknecht’s state of mind on New Year’s Eve, just before prosecutors say the fire was set.

Mr. Rinderknecht was unable to secure plans for the evening and “exhibited extreme anger, indignation and frustration” about that, according to an affidavit. He was upset about his relationship with a co-worker whom he had dated earlier in the year. He shared information about his feelings toward her with ChatGPT more than 50 times, according to the court documents.

On Dec. 30, the woman asked Mr. Rinderknecht for space. He then left her “two manic voice mails,” according to the affidavit. He reached out to two other people to try to make alternate plans, but neither came through.

On New Year’s Eve, he drove several Uber passengers, who later recalled that he had been driving erratically, ranting about Mr. Mangione, capitalism and vigilantism. He dropped off his final passenger in Pacific Palisades and hiked up a trail, where he listened to a French rap song about despair. (The music video for the song shows the singer lighting things on fire, prosecutors note.)

He was obsessed with fire, prosecutors say. A few months before the Palisades fire, he asked ChatGPT to generate images of people running away from a burning forest. On Dec. 5, he viewed images of a wildfire in Southern California caused by arson. On Dec. 29, he filmed fire engines leaving a Hollywood station and said out loud, presumably to himself, “They’re coming for you, bro,” and warned himself to get his mind in order and “not be liking this craziness,” according to the court documents.

Prosecutors say that just after midnight he lit a fire in the chaparral, and then repeatedly called 911. He watched as fire trucks arrived and took videos of their efforts. That blaze, known as the Lachman fire, was not fully put out; it rekindled amid terrible winds seven days later and became known as the Palisades fire, prosecutors say.