Luigi Mangione has pleaded not guilty to federal charges of gunning down health insurance executive Brian Thompson late last year.
His plea came a day after prosecutors formally stated their intent to seek the death penalty.
Appearing in a packed lower Manhattan courtroom on Friday local time, Mr Mangione stood up as US District Judge Margaret Garnett asked for his plea.
"Not guilty," he said.
Mr Mangione, an Ivy League graduate from a prominent Maryland real estate family, also faces separate state murder charges.
He is accused of shooting Mr Thompson, 50, in the back outside a Manhattan hotel on December 4 last year as the executive arrived for UnitedHealthcare's annual investor conference.
The killing and ensuing five-day manhunt leading to Mr Mangione's arrest rattled the business community, with some health insurers hastily switching to remote work.
Claims death penalty announcement 'unapologetically political'
US Attorney General Pam Bondi earlier this month announced the justice department would seek the death penalty for Mr Mangione.
The Manhattan US Attorney's office formalised their intent in a Thursday night court filing.
It follows through on US President Donald Trump's campaign promise to vigorously pursue capital punishment.
Mr Mangione's lawyers have said Ms Bondi's April 1 announcement was "unapologetically political" and breached government protocols for death penalty decisions.
Ms Garnett gave the defence until June 27 to file a legal brief laying out their arguments as to why the government should be barred from pursuing capital punishment.
The judge set Mr Mangione's next court date for December 5, and said she would aim for a trial date some time next year.
She also reminded the lawyers about strict limits on public statements that could impede Mangione's right to a fair trial.
Prosecutors say Mangione presents a 'future danger'
Authorities say the words "deny", "delay", and "depose" were found written on shell casings at the crime scene of Mr Thompson's death.
The phrase echoes tactics some accuse health insurers of using to avoid paying out claims.
While public officials condemned the killing, some Americans have cheered Mr Mangione, saying he drew attention to steep US healthcare costs and the power of health insurers to refuse payment.
In justifying their decision to seek the death penalty, prosecutors wrote in a Thursday night court filing that Mr Mangione "presents a future danger".
They said the 26-year-old has "expressed an intent to target an entire industry" and "rally political and social opposition" to that industry by engaging in an act of lethal violence.
If Mr Mangione is convicted in the federal case, the jury would determine in a separate phase of the trial whether to recommend the death penalty.
Any such recommendation must be unanimous, and the judge would be required to impose it.
ABC/Reuters
