Officers in Pennsylvania said at a press conference that they were working to determine if Mangione had any accomplices and if he intended to kill anyone else. They said he had been in Pennsylvania for several days and were investigating exactly where he was and what he did in the state.

Mangione, a Maryland native, had multiple fraudulent identifications, including a fake New Jersey ID that matched the one used by the gunman to check into a Manhattan hostel days before the shooting, officials said.

Police also found a handwritten document that spoke to “both his motivation and his mindset”, New York City Police Commissioner Jessica Tisch said earlier on Monday.

While the document did not mention specific targets, Mangione harboured “ill will toward corporate America”, said Joseph Kenny, the NYPD’s chief of detectives.

Mangione graduated from a private all-boys school in Baltimore as valedictorian in 2016 before earning dual engineering degrees in 2020 at the University of Pennsylvania, a prestigious Ivy League university, according to school records. His last known address was in Honolulu, officials said.

Thompson, 50, was gunned down outside a Manhattan hotel early on Wednesday morning by a masked man who appeared to wait for his arrival before shooting the executive from behind.

The suspect ran from the scene and then rode a bike into Central Park. Surveillance video captured him exiting the park and taking a taxi to a bus station in northern Manhattan, where police believe he used a bus to flee the city.

Police said Thompson appeared to be deliberately targeted and were investigating whether others also may have been at risk.

The words “deny,” “defend” and “depose” were carved into shell casings found at the scene, several news outlets have reported. The words evoke the title of a book critical of the insurance industry published in 2010 titled Delay, Deny, Defend: Why Insurance Companies Don’t Pay Claims and What You Can Do About It.

A Facebook profile that appears to belong to Mangione identified him as a native of Towson, Maryland, and a former student at the University of Pennsylvania. Photos appear to show Mangione at Stanford University wearing Stanford-branded clothing.

An X account that appears to be owned by Mangione says he lives in Honolulu, Hawaii.

Thompson’s murder unleashed a wave of frustration from Americans who have seen their health insurance claims or care denied, faced unexpected costs or paid more for premiums and medical care – all trends that are rising, according to recent data.

Pennsylvania Governor Josh Shapiro, speaking at a press conference with prosecutors and police in Altoona, said he understood the frustrations that some Americans, angered by health insurance companies and their refusal to pay for some treatments, had expressed online since Thompson’s killing. But he rejected the glorification of the suspect in some circles online.

“In America, we do not kill people in cold blood to resolve policy differences or express a viewpoint,” Shapiro said.

Thompson, a father of two, had been CEO of UnitedHealth Group’s insurance unit since April 2021, part of a 20-year career with the company. He had been in New York to attend the company’s annual investor conference.

“Our hope is that today’s apprehension brings some relief to Brian’s family, friends, colleagues and the many others affected by this unspeakable tragedy,” a spokesperson for UnitedHealth said.