Mangione, 26, stood with his lawyers as he entered the plea, leaning forward toward a microphone as U.S. District Judge Margaret Garnett asked him if understood the indictment and the charges against him.

Mangione said, “yes.” Asked how he wished to plead, Mangione said simply, “not guilty” and sat down.

Mangione’s arraignment for the killing last December attracted several dozen people to the federal courthouse in Manhattan, including former Army intelligence analyst Chelsea Manning, who served about seven years in prison for stealing classified diplomatic cables.

Mangione, who has been held in a federal jail in Brooklyn since his arrest, arrived to court in a mustard-colored jail suit. He chatted with one of his lawyers, death penalty counsel Avi Moskowitz, as they wanted for the arraignment to begin.

Late Thursday night, federal prosecutors filed a required notice of their intent to seek the death penalty.

That came weeks after U.S. Attorney General Pam Bondi announced that she would be directing federal prosecutors to seek the death penalty for what she called “an act of political violence” and a “premeditated, cold-blooded assassination that shocked America.”

It was the first time the Justice Department said it was pursuing capital punishment since President Donald Trump returned to office Jan. 20 with a vow to resume federal executions after they were halted under the previous administration.

Mangione’s lawyers have argued that Bondi’s announcement was a “political stunt” that corrupted the grand jury process and deprived him of his constitutional right to due process. They had sought to block prosecutors from seeking the death penalty.

Mangione’s federal indictment includes a charge of murder through use of a firearm, which carries the possibility of the death penalty. The indictment, which mirrors a criminal complaint brought after Mangione’s arrest also charges him with stalking and a gun offense.

Mangione, an Ivy League graduate from a prominent Maryland real estate family, faces separate federal and state murder charges after authorities say he gunned down Thompson, 50, outside a Manhattan hotel on Dec. 4 as the executive arrived for UnitedHealthcare’s annual investor conference.

The state murder charges carry a maximum punishment of life in prison.

Surveillance video showed a masked gunman shooting Thompson from behind. Police say the words “delay,” “deny” and “depose” were scrawled on the ammunition, mimicking a phrase commonly used to describe how insurers avoid paying claims.

The killing and ensuing five-day search leading to Mangione’s arrest rattled the business community, with some health insurers deleting photos of executives from their websites and switching to online shareholder meetings. At the same time, some health insurance critics have rallied around Mangione as a stand-in for frustrations over coverage denials and hefty medical bills.

Prosecutors have said the two cases will proceed on parallel tracks, with the state case expected to go to trial first, but Mangione lawyer Karen Friedman Agnifilo said his defense team would seek to have the federal case take precedent because it involves the death penalty.

Mangione was arrested Dec. 9 in Altoona, Pennsylvania, about 230 miles (about 370 kilometers) west of New York City and whisked to Manhattan by plane and helicopter.

Police said Mangione had a 9mm handgun that matched the one used in the shooting and other items including a notebook in which they say he expressed hostility toward the health insurance industry and wealthy executives.

Among the entries, prosecutors said, was one from August 2024 that said “the target is insurance” because “it checks every box” and one from October that describes an intent to “wack” an insurance company CEO. UnitedHealthcare, the largest U.S. health insurer, has said Mangione was never a client.