Luigi Mangione has pleaded not guilty to all federal charges brought over the fatal shooting of UnitedHealthcare CEO Brian Thompson in New York last year.
The 26 year old, who was arrested in December and accused of shooting Mr Thompson outside a Manhattan hotel, faces the charges of murder and stalking.
His not guilty plea means he will now face trial and prosecutors are seeking the death penalty if he is convicted.
Mr Mangione arrived at the Lower Manhattan court on Friday wearing a prison outfit and with his hands in cuffs. He acknowledged he had read the indictment against him before entering his plea, telling the judge: "not guilty".
Earlier on Friday, federal prosecutors officially filed to seek the death penalty in this case.
They argued that he carried out Mr Thompson's murder "to amplify an ideological message" and spark resistance to the health insurance industry.
US Attorney General Pam Bondi, who directed prosecutors to seek capital punishment, said in April that Mr Thompson's death was "an act of political violence".
Mr Mangione's lawyers previously called discussion of executing him "barbaric".
During the 35-minute hearing on Friday, Judge Margaret Garnett attempted to co-ordinate a pre-trial schedule, while Mr Mangione's lawyers continued to raise objections to his indictments on both federal and state charges in New York.
The judge agreed Mr Mangione's lawyers would need months to go through prosecutors' "three terabytes" of evidence, including police footage, data from social media, financial and phone companies and other evidence from state prosecutors.
It means Mr Mangione's federal trial will not take place before 2026 - with the judge planning his next federal appearance for 5 December, when a "firm trial date" will be set.
During the hearing, Mr Mangione's lawyer, Karen Friedman Agnifilo, pushed for her client to be tried in federal court - where the death penalty is at stake - before state court, arguing the reverse would raise "constitutional issues".
She also accused state prosecutors of "eavesdropping" on Mr Mangione's recorded calls with her from jail. Judge Garnett asked prosecutors to write a letter within seven days explaining how Mr Mangione would be ensured access to a separate phone line to make privileged calls with his legal team.
The judge also asked Ms Friedman Agnifilo to submit a new motion by 27 June requesting the government be prevented from seeking the death penalty, since she submitted her first motion before prosecutors formally filed notice that they would do so.
Judge Garnett also asked prosecutors to remind Bondi and government officials of rules surrounding public statements and their impact on a fair trial and jury selection.
ReutersMr Mangione is also facing state charges in both Pennsylvania, where he was arrested, and New York. At an arraignment in December, he pleaded not guilty to state murder and terrorism charges in New York.
Mr Thompson was shot dead in Manhattan early on 4 December last year.
The suspect escaped the scene before exiting the city. Five days later, Mr Mangione was arrested at a McDonald's restaurant in Pennsylvania.
Public reaction to Mr Thompson's killing has shed light on deep frustrations with privatised healthcare. Some have celebrated Mr Mangione has a folk hero, and a fund set up for his legal defence garnered nearly $1m (£750,000) in donations.
Supporters gathered outside the courthouse on Friday too.
Shell casings with the words "deny", "defend" and "depose" were found at the crime scene. Critics say these words are associated with healthcare companies avoiding payouts and increasing their profits.
