Luigi Mangione, the suspect accused of murdering UnitedHealthcare CEO Brian Thompson in Manhattan on Dec. 4, pleaded not guilty Monday morning in a New York state court in Manhattan.

Mangione, 26, was indicted on 11 state charges, including first-degree murder, second-degree murder as a crime of terrorism and several weapons charges. If convicted of the most serious charge, he could face life in prison without parole.

The charges include one count of first-degree murder, a AI felony; two counts of second-degree murder, a felony of AI; two counts of second-degree possession of a firearm, a Class C felony; four counts of third-degree possession of a weapon, a Class D felony; one count of Criminal Possession of a Weapon in the Fourth Degree, a Class E felony; and one count of second-degree criminal possession of a forged instrument, a Class D felony.

Mangione’s attorney, Karen Friedman Agnifilo, claimed during the court appearance that statements from government officials such as New York City Mayor Eric Adams jeopardized her client’s right to a fair trial.

MURDER OF UNITEDHEALTHCARE CEO: DEATH PENALTY ON THE TABLE FOR SUSPECT LUIGI MANGIONE, WHO FACE FEDERAL CHARGES

Luigi Mangione enters the courtroom of the New York Criminal Court on Monday, December 23, 2024. Mangione is facing federal charges for the alleged murder of UnitedHealthcare CEO Brian Thompson earlier this month. (Rashid Umar Abbasi for Fox News Digital)

“I am very concerned about my client’s right to a fair trial,” Agnifilo said, raising further concerns about Mangione’s treatment since his arrival in New York City.

“He’s a young man and he’s being treated like a human ping-pong ball,” Agnifilo said between two warring jurisdictions, adding that the overwhelming police presence upon his arrival in New York City on Thursday was the worst perpetrator walk she had ever seen .

Luigi Mangione enters the courtroom of the New York Criminal Court on Monday, December 23, 2024. Mangione is facing federal charges for the alleged murder of UnitedHealthcare CEO Brian Thompson earlier this month. (Rashid Umar Abbasi for Fox News Digital)

“This must stop and my client is entitled to a fair trial and the presumption of innocence,” Agnifilo said, vowing to fight the charges.

Mangione remains in federal custody pending a transfer between state and federal authorities. His next court date was scheduled for February 21.

As Mangione left court with a police escort, he appeared to raise his eyebrows and shake his head in response to a question from the media.

Protesters were seen outside the courthouse chanting “release Luigi” and holding signs reading: “The United States healthcare system stole my livelihood, prosecute malicious profiteers” and “Family Court has blood on their hands.”

Luigi Mangione leaves Manhattan Criminal Court on December 23, 2024 in New York City. Mangione, 26, was charged with murder in the Dec. 4 killing of UnitedHealthcare CEO Brian Thompson on a Manhattan street. (Adam Gray/Getty Images)

Since Mangione’s arrest, both his alleged actions and physical appearance intrigued social media users, who flooded with positive posts about the murder suspect.

New York prosecutors say Mangione planned to travel to New York, find Thompson, a Minnesota resident who was in town for UnitedHealthcare’s annual shareholder conference, and kill him. Mangione allegedly shot Thompson from behind using a 3D-printed ghost gun and a silencer.

Thompson was appointed CEO of UnitedHealthcare in April 2021. He is survived by his wife Paulette “Pauley” Thompson, 51, and their two children.

Prosecutors say Mangione fled New York City within an hour of the crime traveled to Altoona, Pennsylvania.

Attorney Karen Friedman Agnifilo appears at the New York Criminal Court on Monday, December 23, 2024, for the arraignment of Luigi Mangione. Agnifilo is representing Mangione as he faces charges in the killing of UnitedHealthcare CEO Brian Thompson earlier this month. (Rashid Umar Abbasi for Fox News Digital)

Altoona police arrested Mangione five days later, on Dec. 9, at a McDonald’s after she received a tip from an employee. The suspect allegedly presented local police with a fake ID and appeared to shake when they asked if he had recently been to New York.

Officers also found a handwritten manifesto condemning the healthcare industry NYPD Chief of Detectives Joseph Kenny previously told Fox News. The manifesto specifically mentioned UnitedHealthcare.

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While officials did not comment on an official motive, there was public speculation that the suspect had strong grievances with both the healthcare and insurance industries.

Fox News’ Maria Paronich, Grace Taggart and Audrey Conklin contributed to this report.