New York, New York - Luigi Mangione on Monday pleaded not guilty in New York to state charges including the "terrorist" murder of UnitedHealthcare CEO Brian Thompson.

Mangione wore a white shirt under a burgundy sweater, and was shackled during his appearance in a Manhattan court where he was flanked by police officers, an AFP correspondent saw.

Monday's hearing came after the 26-year-old appeared in a New York court last week to face federal charges also following his dramatic extradition by plane and helicopter from Pennsylvania, where he was arrested at a McDonald's restaurant.

Thompson's killing on December 4 brought to the surface deep public frustration with the lucrative US health care system, with many seeing Mangione as a hero and feeling little sympathy for the executive of a company that denied insurance claims more than any other.

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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.

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

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. After checking into a Manhattan hostel with a fake ID, he allegedly scoped out the victim's hotel and the conference venue where the shooting took place.

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