Luigi Mangione admitted on Friday that he shot UnitedHealthcare CEO Brian Thompson in a crime that shook the country, after he appeared in US federal court to change his plea to guilty.

“On the morning of December 4, I shot Mr Thompson in Manhattan, and he died,” Mangione told a federal judge.

When the judge asked him whether he understood that he could spend the rest of his life in prison, he answered, “Yes”.

28-year-old Mangione also said that he stalked the executive to an investor conference in 2024 before shooting him. He claimed that he used a ruse to gather information about the event and contacted the health insurance company ahead of time under the guise of being an investor at the firm.

Mangione said that he pursued Thompson “after years of enduring severe pain from a broken back and navigating the obstacles of the health insurance system”.

In order to avoid the New York state murder charges, Mangione entered federal court and pleaded guilty to stalking. His defence team plans to argue the New York State murder trial should be dismissed on double jeopardy grounds, which means someone shouldn’t be prosecuted twice for the same underlying conduct.

“Mr Mangione has accepted full responsibility for the death of Brian Thompson,” defence lawyer Karen Friedman said outside court, arguing that her client shouldn’t face two prosecutions over a “single tragic event”.

Surveillance video of the ambush showed a masked gunman shooting Thompson, 50, from behind. Police say “delay”, “deny” and “depose” were written on the ammunition, mimicking a phrase used to describe how insurers avoid paying claims.

Supporters of Mangione gathered at the Manhattan courthouse to show their support amid heavy security as a helicopter hovered above.

In June, Mangione’s lawyers said they would pursue a psychiatric defence in the state case but reversed course a day later. The defence, involving claims that he was suffering from extreme emotional disturbance at the time of the killing, isn’t allowed in federal court.