Luigi Mangione was arraigned Friday on federal charges in the December killing of UnitedHealthcare CEO Brian Thompson — a deliberate attack carried out to "amplify an ideological message" and stoke "broad-based resistance" to the health insurance industry, federal prosecutors said in a court filing.

Mangione, 26, appeared in Manhattan federal court a week after a grand jury indicted him on four counts , including murder through use of a firearm, which carries a maximum penalty of death.

Mangione, who pleaded not guilty, appeared calm throughout the 35-minute hearing as he conferred with his attorneys. Unlike past court appearances, he was dressed head-to-toe in a tan prison uniform rather than civilian clothes. He was not handcuffed and appeared to have had a recent haircut.

At one point toward the conclusion of the hearing, Mangione appeared to wink at someone seated in the packed gallery. It was not immediate clear who he was interacting with.

His defense team, led by Karen Friedman Agnifilo, alleged her client is enduring an "unprecedented dual prosecution," referring to state and federal charges Mangione faces in New York. She insisted the overlapping cases created constitutional and logistical issues for the accused murderer.

Friedman Agnifilo also told District Judge Margaret Garnett that prosecutors in Manhattan had inadvertently listened to a telephone call from jail between Mangione and his legal team. She described the incident as "eavesdropping," though she said it appeared to be unintentional and limited to one person.

"This is the very first time we've heard of this situation," federal prosecutor Dominic Gentile told Judge Garnett.

The judge then ordered federal prosecutors to submit a letter within seven days confirming that Mangione will have private phone access to his legal team.

Mangione firmly replied "yes" when Judge Garnett asked him if he understood the charges he faces. He then said "not guilty" when Garnett asked how he wanted to plead.

A medical examiner and experts from phone companies are among the witnesses who will testify at Mangione's trial, Gentile said.

Outside of court, throngs of demonstrators showed up to support Mangione, including some with signs reading "Free Luigi."

Feds Push for Death Penalty

Federal prosecutors filed a notice of intent to seek the death penalty on Thursday, insisting capital punishment is "justified" upon conviction of murder through use of a firearm. Mangione also killed Thompson after "substantial planning and premeditation," prosecutors alleged.

"Luigi Nicholas Mangione intentionally killed Brian Thompson, the chief executive officer of a health insurance company, as the victim walked to an investor conference in Midtown Manhattan," the filing reads. "Mangione elected to murder Thompson under these circumstances to amplify an ideological message, maximize the visibility and impact of the victim's murder, and to provoke broad-based resistance to the victim's industry."

Mangione also represents a "future danger" since he expressed intent to target the health insurance industry and rally "political and social opposition" to insurers by engaging in a deadly act of violence, federal prosecutors said.

"And he took steps to evade law enforcement, flee New York City immediately after the murder, and cross state lines while armed with a privately manufactured firearm and silencer," the filing concludes.

U.S. Attorney General Pam Bondi announced earlier this month she directed federal prosecutors to seek the death penalty in the high-profile slaying, marking the first case in which the federal prosecutors will pursue capital punishment since President Trump returned to the White House.

Bondi characterized Thompson's murder as an "act of political violence" while alleging Mangione and his premeditated plot also posed grave risk of death to additional victims since it occurred in public with bystanders nearby.

Attorneys for Mangione, meanwhile, later alleged in a motion filed on April 11 that Bondi's public statements, including her April 1 press release and an appearance on Fox News , violated "established death penalty protocol" and "indelibly prejudiced" the grand jury process.

"The stakes could not be higher," the April 11 filing reads. "The United States government intends to kill Mr. Mangione as a political stunt."

A Killing That Shocked America

Mangione, a University of Pennsylvania graduate from a prominent Maryland real estate family, allegedly fatally shot Thompson, 50, as the executive arrived at the company's annual investor conference in New York City on December 4.

Responding police recovered 9mm shell casings at the scene, including some inscribed with the words "deny," "delay" and "depose," according to a federal complaint unsealed in December.

Five days later, after some firms reassessed corporate security plans in the wake of Thompson's execution-style slaying, Mangione was arrested at a McDonald's in Altoona, Pennsylvania .

Investigators found Mangione in possessions of a loaded 9mm pistol and silencer consistent with the weapon used to kill Thompson, as well as a notebook, several thousand dollars in cash and a letter addressed "to the feds," according to the complaint.

The notebook contained sections expressing hostility towards the health insurance industry and wealthy executives, with passages indicating the "details are finally coming together" and that the "target is insurance" since "it checks every box," the complaint states.

In one entry from October 22, roughly six weeks before Thompson's murder, the notebook referenced an investor conference and an intent to "wack" an insurance company CEO, according to the complaint.

The letter recovered from Mangione indicated that he "wasn't working with anyone," investigators said.

"This was fairly trivial: Some elementary social engineering, basic CAD, a lot of patience," the entry read, likely referring to computer-aided design. "P.S. you can check serial numbers to verify this is all self-funded. My own ATM withdrawals."

The accused killer has received widespread public support among some circles, including critics of insurance companies and well-paid corporate executives. As of early Friday, an online fundraiser for Mangione had eclipsed $948,000.

Mangione, who remains held without bond at the Metropolitan Detention Center in Brooklyn, has pleaded not guilty to state charges in New York, including murder as an act of terrorism.

He's due to return to court in the state case on June 26. His next court date on the federal charges has been set for December 5, at which point a trial date is likely to be set for sometime in 2026.