The suspect accused of shooting dead the chief executive of UnitedHealthcare has pleaded not guilty to 11 murder and terror charges.
Prosecutors say Luigi Mangione shot dead Brian Thompson as he was walking to an investor conference in Manhattan, New York, on the morning of 4 December.
The 26-year-old was formally charged last week by the Manhattan district attorney with multiple counts of murder, including murder as an act of terrorism.
A small group of protesters gathered outside the courtroom earlier in sub-zero temperatures, waving signs expressing support for the defendant and anger at healthcare companies.
One placard read "DENY, DEFEND, DEPOSE", a phrase police say had been etched onto shell casings at the crime
scene.
The words reflect tactics insurers are accused of employing to avoid paying out on claims to US patients.
Despite facing terrorism charges, Mangione has been lauded as a folk hero by some Americans who despair at the enormous healthcare costs and power of insurers to refuse to pay for some treatments.
Read more on Mangione:
Hero or villain: Everyone has thoughts on alleged killer
The suspect was detained in a Pennsylvania McDonald's after a five-day search, carrying a gun that matched the one used in the shooting and a fake ID, police said.
He was extradited from Pennsylvania on Thursday and quickly rushed to New York City.
As well as this state trial, he also faces federal charges. The two cases will proceed on parallel tracks, prosecutors have said.
The state charges allege Mangione intended to "intimidate or coerce a civilian population" and influence
policy, while the federal charges accuse him of stalking and killing someone.
According to federal prosecutors, the police who arrested him also found a notebook containing writing that expressed "hostility towards the health insurance industry and wealthy executives in particular".
Mangione is being held in a Brooklyn federal jail alongside several other high-profile defendants, including rapper Sean "Diddy" Combs and Sam Bankman-Fried, co-founder of failed crypto exchange FTX.
(c) Sky News 2024: Luigi Mangione pleads not guilty to murder and terror charges over healthcare CEO's killing
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