They came in hoodies, they came in masks, shuffling their feet and laughing nervously while waiting for a winner to be announced.

Just a few days after UnitedHealthcare CEO Brian Thompson was murdered on a New York City sidewalk, these young men had lined up in Washington Square Park to compete in a lookalike contest for the man wanted for his murder.

It was sparsely attended and seen as a joke by those who did turn up, said Talia Jane, a journalist who was there.

But it underlined an obsession with a murder suspect that has gripped social media since the killing on 4 December, fuelled by latent anger directed at America's private health insurers.

"There was a lot of tinder already there, a lot of discontent, a lot of frustration already there, and [this] sort of threw a match on it," Ms Jane said.

And it has only grown since the suspect was named as Luigi Mangione, a 26-year-old Ivy League-educated member of a prominent Baltimore family.

In TikTok videos, memes and group chats, a young man accused of shooting a father-of-two in the back on a New York City sidewalk has been fawned over and praised as a kind of folk hero.

This fetishisation was remarkably widespread, not limited to radical corners of the internet or any political affiliation, troubling many observers.

"We do not kill people in cold blood to resolve policy differences or express a viewpoint," said Josh Shapiro, the governor of Pennsylvania, where Mr Mangione was arrested at a McDonald's.

"In a civil society, we are all less safe when ideologues engage in vigilante justice."

Almost immediately after Mr Thompson was shot dead, the internet began to lionise his suspected killer. On TikTok, people posted videos of a "CEO assassin" New York City walking tour. On Spotify, playlists dedicated to the suspect started to appear.

Once Mr Mangione was arrested, these fans came to his defence.

The start of his legal battles prompted anonymous donors to chip in thousands of dollars towards his defence through various online fundraisers.

Etsy was flooded with pro-Mangione apparel, while Amazon pulled similar products from their site.

The McDonald's worker alleged to have turned him in has become a target for online hate, while the fast-food franchise itself has been spammed with bad reviews.

The police department in Altoona, Pennsylvania, that arrested him even received death threats.

Much of this online reaction has focused on his looks, with the internet dubbing him the "hot assassin".

Indeed, Mr Mangione's appearance, which he showed off in shirtless social media posts, is now clearly part of the appeal, said cultural critic Blakely Thornton.

Americans are effectively "programmed" to trust and empathise with men who look like Mr Mangione, he said.

"That's why they are the protagonists in our movies, books and stories."

Public adoration for handsome men accused of crimes is not new - from Ted Bundy to Jeremy Meeks, violent men have developed cult followings.

But Professor Tanya Horeck, an expert on digital culture and true crime from Anglia Ruskin University, says that social media has given those sentiments massive visibility, and helped them spread.

The internet has caused "a blurring of the lines between celebrity and criminality", she told the BBC, adding that when people see a good-looking person pop into their feeds, their first thought is lust, not moral criticism.

"The mood around Luigi Mangione is 'thirst'," she said.

Beyond his appearance, a large part of Mr Mangione's online appeal is clearly his apparent ire against the private healthcare industry and corporate elites in general. US media has reported that Mr Mangione was arrested carrying a handwritten document that said "these parasites had it coming".

The Network Contagion Research Institute (NCRI), a non-profit extremism research group based in New Jersey, said that after the shooting the hashtag #EatTheRich went viral.

Since Mr Mangione's arrest, variations of "#FreeLuigi" were posted on X over 50,000 times, likely getting tens of millions of impressions. And by some measures, the NCRI said, engagement with posts about Mr Thompson's killing across platforms like X, Reddit, and others surpassed that of the assassination attempt against Donald Trump in July.

An analysis of a sample of comments carried out by market research firm OneCliq found the vast majority - four-fifths - contained criticism of the healthcare system.

Mr Mangione's X account has gained more than 400,000 followers since the shooting.

The shooting also seemed to inspire others to take action against healthcare insurers - "wanted" posters of other CEOs appeared around New York City, and a woman in Florida was arrested after telling an insurance agent on the phone "Delay, Deny, Depose. You people are next," alluding to the words inscribed on bullet casings found at the murder scene.

Alex Goldenberg, a senior adviser at the NCRI, called the online reaction a "turning point" and "a catalyst for the normalisation of political violence that was once confined to extremists on the fringes".

He compared the wave of comments to the online activity following racist mass murders, designed to defend the killers and signal-boost their beliefs - only more widespread, and happening across mainstream social media networks.

"The dynamic we are observing is eerily similar to the activity on platforms like 4chan, 8chan, Discord, and in other dark corners of the internet, where mass shootings are often met with glee," he said.

Tim Weninger, a computer science professor at Notre Dame and expert in social media and artificial intelligence, said evidence suggests that the groundswell was authentic - not powered by bots or government influence operations.

"People are pissed off at the healthcare industry and they are using social media to express their frustrations," he said. "They're expressing those frustrations by supporting this suspect."

Recent research by Commonwealth Fund, a health policy institute, found 45% of insured working-age adults were charged for something they thought should have been free or covered by insurance, and 17% of respondents said their insurer denied coverage for care that was recommended by their doctor.

There are indications that the shooting has prompted some introspection on the part of healthcare companies.

"I think all of us are taking a step back and trying to understand what's happening with patients and their experiences," Pfizer's chief sustainability officer Caroline Roan told a conference in New York on Wednesday, according to Reuters.

Some of the people who have been protesting against health insurance companies for years, before online criticism about UHC took off, understand some of the darker sentiments, even if they don't endorse them.

"It's a horrific act of violence, and I absolutely condemn it no matter the motivation," Jenn Coffey, who has been fighting to get UHC to cover her medical bills, said of Mr Thompson's killing. "But I'm not shocked by the reaction."

Ms Coffey, 53, from Manchester, New Hampshire, was diagnosed with breast cancer in 2013 and later fell ill with complex regional pain syndrome, a potentially debilitating neurological condition. When a doctor suggested that she try ketamine therapy, her UHC insurance would not cover the procedure, she said. She has since become involved in a protest campaign organised by activist group People's Action.

Ms Coffey's illness forced her to stop working as an emergency medical technician. She said she started a crowdfunding drive and had to sell most of her belongings in order to pay for the treatment herself.

"I get to have some normality. I can sit up and paint, or I can enjoy a meal with my family" because of the treatment, she said. "I can have a life that's worth living."

UnitedHealth Group told the BBC they could not comment on individual cases for privacy reasons.

At the same time, much of the online discussion has effectively ignored the victim, Brian Thompson, who was 50.

"It's incredibly bleak that [Thompson's death] hasn't been covered as much because, bottom line, a person died, a person was murdered," Blakely Thornton said.

"The collective rage over [the healthcare industry] is really outweighing what is still a tragedy."

In his last post on LinkedIn, Mr Thompson talked about efforts to make healthcare more affordable - and was criticised in the comments. CBS News, the BBC's US partner, obtained a message that UnitedHealth CEO Andrew Witty sent to staff this week, memorialising Mr Thompson and calling his murdered colleague "one of the good guys".

"He was certainly one of the smartest guys. I think he was one of the best guys. I'm going to miss him. And I am incredibly proud to call him my friend," he wrote.

In the email, the company shared messages from customers, including one who wrote about recovering from cancer, saying their treatment was paid for by the insurance company's benefits.

"I'm thankful to UHC and everyone there who works within a broken system to help as many people as they can," they wrote, according to the company.

Another message said: "So very sad that this world is so hateful. I have always had great experiences with UHC."

Ms Coffey, the UHC policyholder and patient, said: "My heart goes out to the family and I can't imagine what they're having to struggle with that [killing]. It's awful to me that this is the catalyst for this debate."

"I would have much rather sat down and had a conversation with him."

With additional reporting from Grace Dean