Mr. Mangione appeared in court on Monday wearing a burgundy sweater, a white collared shirt and khaki pants.
In addition to a long stream of reporters waiting for the suspect to appear, members of the public – almost all young women – were in court, some of whom told CBS, the BBC’s American partner, that they were there to show their support.
Mr. Mangione faces 11 New York State felony charges, including first-degree murder and murder as a crime of terrorism.
If convicted on all counts, he faces a mandatory sentence of life in prison without the possibility of parole.
Federal prosecutors also separately charged Mr. Mangione with using a firearm to commit murder and stalking in the state resulting in death. Both charges could make him eligible for the death penalty.
He has yet to plead guilty to those charges.
Prosecutors said the federal and state cases will proceed in parallel with each other.
In court last week, Mr. Mangione’s lawyer, Karen Friedman Anyifila, said the two sets of charges conflicted: the state charges accused him of seeking to “intimidate or coerce the civilian population,” while the federal charges focused on crimes against individuals. .
Ms Anyifila said the overlapping cases were “disturbing” and “very unusual”.
“I’ve never seen anything like what’s going on here” in 30 years of practicing law, she said.
In court Monday, she also told the judge that she believed the statements made by public officials — including New York City Mayor Eric Adams — made her “very concerned about my client’s right to a fair trial.”
– This is a young man, – she said. “He’s being treated like a human ping-pong ball between warring jurisdictions.”
The judge, Gregory Caro, said he could not control what happened outside the court, but promised that Mr. Mangione would receive a fair trial.
The suspect is currently being held at the Metropolitan Detention Center (MDC) in Brooklyn after being returned to New York last week under tight security.
Authorities believe that Mr. Mangione carried out the targeted killing of Mr. Thompson, pointing to evidence that he was angry with the US health care industry.
The federal complaint notes that the notebook found in Mr. Mangione’s possession expressed “hostility toward the health insurance industry and specifically toward wealthy executives.”
Some on social media praised Mr. Mangione’s alleged wrongdoing, often sharing their own anger at the US’s private health care system.
Speaking to CBS, the BBC’s US affiliate, on Sunday, Homeland Security Secretary Alejandro Mayorkas said the rhetoric online was “extremely worrying”.
“It speaks to what is really bubbling here in this country,” he said. “And unfortunately, we see that manifested in the violence, the domestic violent extremism that exists.”
