Supporters of Luigi Mangione are raising tens of thousands of dollars for his defense fund.

Luigi Mangione appeared in court on Monday in shackles and under heavy guard to plead not guilty in the murder of UnitedHealthcare CEO Brian Thompson.

Luigi Mangione appeared Monday morning before Judge Gregory Carro. The arraignment is in regard to 11 charges filed against him in the state of New York.

The alleged killer of UnitedHealthcare CEO Brian Thompson was extradited from Pennsylvania to New York City.

Protesters from all walks of life assembled outside the courthouse to show their support for Luigi Mangione

Mangione appeared on Monday in a Manhattan court for his arraignment on state murder and terror charges in the fatal shooting of UnitedHealthcare CEO Brian Thompson.

As investigators continue probing the United Healthcare CEO killing, there is concern among about the long-term impacts the public assassination could cause.

Luigi Mangione is due in a New York state court, where he will face charges in an 11-count indictment accusing him of murdering UnitedHealthcare CEO Brian Thompson “in furtherance of terrorism.”

ABC News’ Aaron Katersky and legal contributor Brian Buckmire discuss the court appearance for the suspect charged in the fatal shooting of UnitedHealthcare CEO Brian Thompson.

Luigi Mangione waived extradition on Thursday and was transported from Pennsylvania to New York.

Luigi Mangione is expected to face New York State murder and terrorism charges on Monday in a Manhattan court room.

Luigi Mangione, the man accused of gunning down UnitedHealthcare CEO Brian Thompson, is charged in a four-count federal criminal complaint.

The 26-year-old accused of shooting UnitedHealthcare CEO Brian Thompson arrived from Pennsylvania on Thursday. Mangione, who could now face the death penalty, has become an unlikely hero to some.

ABC News’ Linsey Davis speaks to criminal defense attorney Arthur Aidala on the criminal complaint filed today against Luigi Mangione and what happens next in the high-profile case.

Prosecutors laid out evidence against the 26-year-old accused in UnitedHealthcare CEO Brian Thompson's fatal shooting, after he arrived from Pennsylvania on Thursday. Mangione has not entered a plea.

Luigi Mangione faces four federal charges, including stalking, a firearms offense involving a silencer and murder through use of a firearm, a charge that makes him eligible for the death penalty.

Luigi Mangione, the man accused of gunning down UnitedHealthcare CEO Brian Thompson, waived extradition on Thursday morning.

ABC News’ Aaron Katersky and ABC News legal contributor Brian Buckmire discuss the federal charges against the suspect in the killing of UnitedHealthcare CEO Brian Thompson.

The suspect in the killing of UnitedHealthcare CEO Brian Thompson will “go forth with New York to await trial or prosecution for his homicide and related charges in New York.”

Mangione, the suspect in the killing of UnitedHealthcare CEO Brian Thompson, is set to return to court in Pennsylvania and is expected to waive extradition to New York, according to his attorney.

Luigi Mangione, the man accused of gunning down UnitedHealthcare CEO Brian Thompson, will waive extradition and could be returned to New York as soon as Thursday.

If his extradition paperwork goes through, the man accused of gunning down UnitedHealthcare CEO Brian Thompson will be cleared to travel from Pennsylvania to New York as early as Thursday.

Luigi Mangione, the man accused of gunning down UnitedHealthcare CEO Brian Thompson, will waive extradition to New York when he appears in court in Pennsylvania on Thursday.

Mangione was indicted in New York for the murder of UnitedHealthcare CEO Brian Thompson and the grand jury has upgraded charges to first-degree murder in furtherance of terrorism.

The man charged with murder for gunning down UnitedHealthcare CEO Brian Thompson was arrested and charged after the shooting outside a Hilton Hotel in Midtown Manhattan.

Luigi Mangione, the accused killer of UnitedHealthcare CEO Brian Thompson, is expected to waive extradition when he appears in court in Pennsylvania on Thursday.

Manhattan District Attorney Alvin Bragg said the Dec. 4 shooting of UnitedHealthcare CEO Brian Thompson was "intended to evoke terror." The 26-year-old remains in custody in Pennsylvania.

Luigi Mangione has been indicted in New York for the murder of Brian Thompson and the grand jury has upgraded charges to first-degree murder in furtherance of terrorism, prosecutors announced.

The accused killer of UnitedHealthcare CEO Brian Thompson is expected to waive extradition from Pennsylvania, where he was arrested a week ago, sources told ABC News.

Luigi Mangione was taken into custody in Pennsylvania on Monday following a dayslong manhunt for the suspect in the fatal shooting of UnitedHealthcare CEO Brian Thompson.

Sources tell ABC News the accused killer of UnitedHealthcare CEO Brian Thompson is almost certain to waive extradition from Pennsylvania, where he was arrested a week ago.

Luigi Mangione's mother filed a missing persons report about her son in San Francisco 16 days before he allegedly shot dead UnitedHealthcare CEO Brian Thompson.

Luigi Mangione, suspect in the murder of the UnitedHealthcare CEO, has hired high-profile defense attorney Karen Friedman Agnifilo, a former prosecutor with the Manhattan district attorney's office.

Mangione retained the services of Manhattan defense attorney Karen Friedman, a longtime veteran of the Manhattan District Attorney’s Office.

Luigi Mangione -- the alleged killer of UnitedHealthcare CEO Brian Thompson -- has been greeted by a surge of enthusiastic online support.

Mangione’s mother had filed a missing persons report about her son on Nov. 18.

Luigi Mangione may waive extradition from Pennsylvania to New York City to face murder charges for killing of UnitedHealthcare CEO Brian Thompson, according to Manhattan District Attorney Alvin Bragg.

Luigi Mangione may waive extradition to New York City as early as Tuesday, Manhattan District Attorney Alvin Bragg said on Friday.

Luigi Mangione has been charged with second-degree murder in the killing of UnitedHealthcare CEO Brian Thompson.

ABC News’ Aaron Katersky reports on the probe into Luigi Mangione, the suspect accused of killing UnitedHealthcare CEO Brian Thompson, who is fighting extradition.

Sources tell ABC News that prosecutors have started presenting evidence against Mangione to a grand jury, seeking an indictment that could strengthen their case for his extradition from Pennsylvania.

In Pennsylvania, Luigi Mangione faces charges for allegedly possessing an untraceable ghost gun. In New York, he is charged with second-degree murder.

The firearm found on the suspect in the murder of the UnitedHealthcare CEO is believed to have been an untraceable ghost gun, according to police.

Police investigators say they have matched the gun to three shell casings found at the scene and matched Luigi Mangione’s fingerprints to prints found near where Brian Thompson was gunned down.

ABC News’ Aaron Katersky discusses the legal moves as Manhattan prosecutors work to secure an indictment against Luigi Mangione for the killing of UnitedHealthcare CEO Brian Thompson.

The ghost gun allegedly in Luigi Mangione's possession when he was arrested has been matched to three shell casings recovered at the crime scene in New York.

"Wanted" signs naming executives were posted in Manhattan this week, NYPD said.

Luigi Mangione was charged in Pennsylvania with carrying a gun without a license, forgery, falsely identifying himself to police and possessing "instruments of crime."

The suspect charged in the fatal shooting of UnitedHealthcare CEO Brian Thompson was arrested in Altoona, Pennsylvania.

Luigi Mangione, who is accused of killing Brian Thompson in New York, could face additional charges, according to Manhattan District Attorney Alvin Bragg.