The 26-year-old was charged with Pennsylvania gun and forgery offences and detained without bail. His Pennsylvania lawyer has questioned the evidence for the forgery charge and the legal grounding for the gun charge. The lawyer also said Mangione would fight extradition to New York.

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Mangione has two court hearings scheduled for Thursday in Pennsylvania, including an extradition hearing, Bragg noted.

Hours after his arrest, the Manhattan district attorney’s office filed paperwork charging him with murder and other offences. Wednesday’s indictment builds on that paperwork.

Investigators’ working theory is that Mangione, an Ivy League computer science grad from a prominent Maryland family, was propelled by anger at the US healthcare system.

A law enforcement bulletin obtained by the Associated Press last week said that when arrested, he was carrying a handwritten letter that called health insurance companies “parasitic” and complained about corporate greed.

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Mangione repeatedly posted on social media about how spinal surgery last year had eased his chronic back pain, encouraging people with similar conditions to speak up for themselves if told they just had to live with it.

In a Reddit post in late April, he advised someone with a back problem to seek additional opinions from surgeons and, if necessary, say the pain made it impossible to work.

“We live in a capitalist society,” Mangione wrote. “I’ve found that the medical industry responds to these keywords far more urgently than you describing unbearable pain and how it’s impacting your quality of life.”

He was never a UnitedHealthcare client, according to the insurer.

Mangione apparently cut himself off from his family and close friends in recent months. His family reported him missing to San Francisco authorities in November.

Brian Thompson, who grew up on a farm in small-town Iowa, was trained as an accountant. A married father of two high-schoolers, he had worked at the giant UnitedHealth Group for 20 years and became chief executive of its insurance arm in 2021.

His killing kindled a fiery outpouring of resentment toward US health insurance companies, as Americans swapped stories online and elsewhere of being denied coverage, left in limbo as doctors and insurers disagreed, and stuck with sizeable bills.

The shooting also rattled senior executives, as “wanted” posters with other healthcare executives’ names and faces appeared on New York streets, and an outpouring of online vitriol prompted police to warn that there could be an “elevated threat”.