New York, New York - A Manhattan judge dismissed terrorism charges against Luigi Mangione, the alleged shooter of UnitedHealthcare CEO Brian Thompson, and set a start date of December 1 for his trial.

Mangione appeared in court wearing a beige jumpsuit and shackles, flanked by armed police and court officers, stopping to smile and nod at a young woman in the front row as he was led out, AFP saw.

"I think we agree that hearings in this case will start December 1," judge Gregory Carro said.

Supporters of Mangione gathered both inside and outside court to show their backing for the 27-year-old, who has become something of a folk hero amid deep public frustration with the lucrative US health care system.

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He is charged in both state and federal court in the December 4 shooting of Thompson.

If convicted in the state case, Mangione could face life imprisonment with no parole. In the federal case, he could technically face the death penalty.

At Tuesday's hearing, the judge said that the grand jury in the case had been presented with insufficient evidence to support the first-degree murder charges, and murder as an act of "terrorism" charge. The remaining counts still stand – including second-degree murder.

One of the protesters, who were mostly young women, held up an Italian flag emblazoned with the words "healthcare is a human right."

Mangione's attorney Karen Friedman Agnifilo has sought clarity on how simultaneous federal and state charges would work, calling the situation "highly unusual."

Dozens of Mangione's supporters chased his legal team along the street after they left court following the packed hearing.

Mangione was arrested in Altoona, Pennsylvania, on December 9 following a tip from staff at a McDonald's restaurant, after a days-long manhunt.

He had traveled to New York by bus from Atlanta about 10 days before the crime, the Department of Justice said.

Early on December 4, Mangione allegedly tracked Thompson, walked up behind him and fired several gunshots from a pistol with a silencer, before fleeing on a bike, the DOJ said.