The suspect accused of killing UnitedHealthcare CEO Brian Thompson has pleaded not guilty to New York state murder and terrorism charges.

Luigi Mangione, 26, appeared in court on Monday to be arraigned on 11 state criminal counts, including murder a crime of terrorism.

He also faces federal stalking and murder charges that could lead to a death penalty sentence.

Prosecutors allege that Mr Mangione shot Mr Thompson in central Manhattan before going on the run. Authorities later arrested him at a McDonald's in Pennsylvania.

Mr Mangione appeared in court on Monday wearing a maroon sweater, white-collared shirt and khaki pants.

In addition to a long stream of journalists waiting for the suspect to appear, members of the public - almost all of them young women - were in court, some of whom told CBS, the BBC's US partner, that they were there to show their support.

Mr Mangione is facing 11 state criminal counts in New York, including first-degree murder and murder as a crime of terrorism.

If convicted of all the counts, he would face a mandatory sentence of life in prison without the possibility of parole.

Federal prosecutors have also separately charged Mr Mangione for using a firearm to commit murder and interstate stalking resulting in death. Both charges could make him eligible for the death penalty.

He has yet to enter a plea on those charges.

Prosecutors have said the federal and state cases will move forward parallel with one another.

In court last week, Mr Mangione's lawyer - Karen Friedman Agnifilo - said that the two sets of charges appear to conflict, with the state charges accusing him of seeking to "intimidate or coerce a civilian population" while the federal charges focus on crimes against an individual.

Ms Agnifilo said that the overlapping cases were "confusing" and "highly unusual".

"I've never seen anything like what is happening here" in 30 years of practicing law, she said.

In court on Monday, she further told the judge that she believed that statements from government officials - including New York City Mayor Eric Adams - make her "very concerned about my client's right to a fair trial".

"This is a young man," she said. "He is being treated like a human ping-pong ball between warring jurisdictions here."

The judge, Gregory Carro, said that he is unable to control what happens outside court, but vowed that Mr Mangione would receive a fair trial.

The suspect is currently in federal custody at the Metropolitan Detention Center (MDC) Brooklyn after being returned to New York amid heavy security last week.

Authorities believe that Mr Mangione carried out a targeted killing of Mr Thompson, pointing to evidence that he was angry at the US healthcare industry.

The federal complaint notes that a notebook found in Mr Mangione's possession expressed "hostility towards the health insurance industry and wealthy executives in particular."

Some on social media praised Mr Mangione's alleged crimes, often sharing their own anger at the US private healthcare system.

Speaking to CBS, the BBC's US partner, on Sunday, Homeland Security Secretary Alejandro Mayorkas said that the online rhetoric has been "extraordinarily alarming".

"It speaks of what is really bubbling here in this country," he said. "And unfortunately we see that manifested in violence, the domestic violent extremism that exists."