Luigi Mangione has admitted killing UnitedHealthcare boss Brian Thompson, pleading guilty to two federal charges related to the fatal shooting in 2024.
Mangione, 28, now faces up to a lifetime in prison when he is sentenced in December.
He also faces a slew of separate charges, including second degree murder, in a state trial beginning next month.
His admission today may throw a wrench in the state trial, as Mangione's lawyers argue he cannot be prosecuted twice for the same crime.
Mangione pleaded guilty to two federal charges of stalking resulting in death on Friday.
"I shot Mr Thompson in Manhattan and he died," Mangione told a Manhattan courtroom, changing his initial plea of not guilty.
Federal prosecutors said the 28-year-old travelled across state lines and used surveillance to kill the healthcare boss.
The charges carry a maximum penalty of life in prison without parole.
Mangione has not made a plea deal with the prosecution, as US media previously reported was a possibility.
Plea deals, which do not exist in the UK, often include an exchange where the defendant gets a lighter sentence, or charges are dropped in exchange for a guilty plea.
Prosecutors have recommended Mangione serve between 24 and 30 years in prison. But that is not binding and Judge Margaret Garnett said in court that she ultimately decides his sentence. The maximum sentence is life in prison and there is no parole in the federal system.
Mangione faced two separate criminal prosecutions: a federal case and another in New York state that is scheduled to start on 8 September.
Though he pleaded guilty in federal court, he still faces eight state charges, including second degree murder, all of which he has pleaded not guilty.
Since his statements in federal court can be used in the upcoming state trial, his defence options are limited.
"In essence, his statements that he followed and shot Mr Thomson are effectively a confession to the crimes in state court too," said Mark Chutkow, a defence attorney and former federal prosecutor.
