In New York, "Wanted" posters with the faces of CEOs have appeared on walls. Websites are selling Mangione merchandise, including hats with "CEO Hunter" printed across a bullseye. And some social media users have swooned over his smile and six-pack abs.

Mangione has been charged with murder for the killing of UnitedHealthcare CEO Brian Thompson, a father of two, in a brazen shooting on Dec. 4 outside a Manhattan hotel before an industry conference, setting off a five-day manhunt for the masked assailant.

The crime he is accused of has been broadly condemned, but the Ivy League educated, photogenic 26-year-old has become an unsettling mixture of folk hero, celebrity, and online crush in certain circles. His support has only seemingly intensified since his arrest on Monday.

Most of the messages on the crowd-sourced fundraising site GiveSendGo reflect a deep frustration shared by many Americans over the US healthcare system - where some treatments and reimbursements can be denied to patients depending on their insurance coverage - as well as broader anger over rising income inequality and soaring executive pay.

"Denying healthcare coverage to people is murder, but no one gets charged with that crime," one donor wrote, calling the killing a "justifiable homicide."

Several others simply wrote, "Deny, Defend, Depose" – the words reportedly written on the shell casings found at the murder scene and intended to invoke tactics some accuse insurers of using to avoid paying out claims.

More than $31,000 had been raised as of Wednesday on GiveSendGo alone.

Felipe Rodriguez, a former NYPD detective sergeant, expressed dismay at the reaction.

"They've made him a martyr for all the troubles people have had with their own insurance companies," said Rodriguez, now an adjunct professor at the John Jay College of Criminal Justice in New York. "I mean, who hasn't had run-ins with their insurance? But he's a stone-cold killer."

Mangione is being held in Pennsylvania on gun and forgery charges while prosecutors in New York seek his extradition. His lawyer said he plans to plead not guilty to the Pennsylvania charges.

Frustration and anger

On Wednesday, New York City Police Commissioner Jessica Tisch said police have matched a gun found on Mangione with the shell casings recovered at the scene and his fingerprints with a water bottle and energy bar wrapper found nearby.

Other evidence includes handwritten documents found in his possession casting his alleged crime as a legitimate response to what he viewed as corporate greed, some media outlets have reported.

Mangione lashed out himself on Tuesday as he was led into a courthouse, shouting in part, "...completely out of touch and an insult to the intelligence of the American people!"

Americans pay more for health care than residents of any other country, and data shows spending on insurance premiums, out-of-pocket costs, pharmaceuticals and hospital services has all increased over the last five years.

Health insurers like UnitedHealth most often manage health benefits on behalf of employers and the government, which have a say in what services and drugs are covered.

Mangione suffered from chronic back pain that impacted his daily life, according to friends and social media posts, though it is unclear whether his personal health played a role in the shooting.

"It's hard to underestimate the anger and angst people have with their insurance companies," said David Shapiro, a former FBI agent and a professor at the John Jay College of Criminal Justice in New York.

Shapiro said he had never seen anything like the reaction to Mangione, but added: "It's not so farfetched given the mood of the country and the ease of cheering this anonymously on the Internet."

On TikTok, users shared videos and photos of Mangione's outburst with messages of praise, such as "this man is an absolute legend" and "class consciousness is rising."

Several sites were selling T-shirts bearing his face with messages such as "FREE LUIGI" and "In This House, Luigi Mangione Is A Hero, End of Story." Others sold hats with the phrase "Don't Deny My Coverage."

'Deeply disturbing'

Support was by no means universal, however.

Several commentators on social media noted Mangione's privileged background as a member of a prominent Baltimore, Maryland family, as compared to Thompson's working class upbringing in rural Iowa, and said the murder was an example of how anti-capitalist rhetoric can incite violence. Others described how their health insurance plans paid for life-saving treatment.

On Wednesday, UnitedHealth Group Inc CEO Andrew Witty sent employees a letter praising Thompson. "Brian was one of the good guys," he wrote. "I'm going to miss him. And I am incredibly proud to call him my friend."

Pennsylvania Governor Josh Shapiro condemned those valorizing Mangione on Monday, calling the response "deeply disturbing."

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

At a panel at the Reuters NEXT conference in New York on Wednesday, executives from Pfizer and Amazon said health care companies are taking a step back to better understand patients' experiences.

"Our health system needs to be better ... There's a lot of things that should cause a lot of outrage," Amazon Pharmacy Chief Medical Officer Vin Gupta said. "It's also true that [the killing] should not have happened. There cannot be this false moral equivalence in our discourse."