Luigi Mangione has admitted to killing UnitedHealthcare CEO Brian Thompson after pleading guilty to federal charges of interstate stalking resulting in death.

"I shot Mr Thompson in Manhattan and he died," the 28-year-old admitted in Manhattan federal court on Friday.

Thompson, a 50-year-old father of two, was shot from behind on 4 December, 2024 as he walked into a hotel in midtown Manhattan. Following the plea hearing, his family issued a statement calling it "an important step toward justice for Brian and for our family".

Mangione faces a maximum sentence of life in prison. He also faces state charges, although his lawyers are seeking to get those dismissed following his guilty plea.

Mangione had initially pleaded not guilty in both the federal and state cases against him for the shooting death of Thompson.

At a hearing in Manhattan federal court on Friday, Mangione's lawyers announced that he intended to plead guilty to the two federal charges against him.

Wearing a beige prison outfit, Mangione appeared solemn and serious throughout the hearing.

Reading a statement aloud to the court, he admitted to planning the attack ahead of time, crossing state lines with the intent to kill and using a gun he printed on a 3-D-printer and equipped with a silencer and magazine.

He told the court he was driven to kill Thompson because of his experience with the healthcare system "after years of enduring severe pains from a broken back".

Mangione stated that he had learned UnitedHealthcare would hold an annual investor conference in Manhattan in December 2024.

He then travelled to New York for the event, he told the court.

He also said he emailed UnitedHealthcare prior to the conference "posing as an investor" and asking questions about the event.

"I emailed UnitedHealthcare with the intent to shoot Brian Thompson," Mangione said in his statement. "On the morning of Dec. 4, 2024 I shot Mr. Thompson in Manhattan in his back."

The sentencing for the federal charges is scheduled for 18 December.

Thompson's family said in a statement that they will continue to seek justice in both New York state and Pennsylvania, where Mangione was arrested.

"While nothing will ease the pain of losing him, we are grateful that the federal justice system has held the person responsible for this heinous act accountable.

"Now we look to the court to ensure that sentencing reflects the severity of this crime," the family added.

Prosecutors have recommended Mangione serve between 24-30 years in prison. But Judge Margaret Garnett said in court that she ultimately decides his sentence.

Garnett explained to Mangione that the recommendation is not binding and that the maximum sentence is life in prison along with financial penalties. There is no parole in the federal system.

Mangione's guilty plea could now throw a wrench into his New York state case.

With the trial slated to begin in September, he faces eight state charges including second degree murder, possession of a forged instrument (for alleged fake driver's license), and six counts of criminal possession of a weapon.

On the steps of the court on Friday, his lawyers argued that because he already pleaded guilty to federal charges, the state case runs afoul of double jeopardy – a constitutional protection that prevents an individual from being prosecuted twice for the same crime.

"New York law does not permit a person to be prosecuted and punished twice for the exact same crime," his lawyer Karen Friedman Agnifilo said.

Friedman Agnifilo also said that, in making a guilty plea in federal court, Mangione has accepted "full responsibility" for the death of Brian Thompson.

But the Manhattan District Attorney's office says it is prepared to go ahead with the state prosecution.

"We are encouraged that Mr Thompson's family received a measure of accountability today," the statement said. "While federal sentencing is pending, we are prepared to litigate the defence motions. The Manhattan DA's Office remains committed to seeking justice for Mr Thompson and his family."