Accused killer Luigi Mangione’s fingerprints have been matched to those near the crime scene in New York where an insurance boss was gunned down.

New York Police Department confirmed on Thursday (AEDT) that fingerprints were taken from a water bottle and a protein bar wrapper found near the murder location.

The clue is the first positive forensic match linking Mangione to the “brazen” shooting of UnitedHealthcare CEO Brian Thompson.

A gun the 26-year-old was carrying when he was arrested has also been matched to three shell casings – which were carved with the words ‘Deny, Defend, Dispose’ – discovered outside the Hilton Hotel.

US media are also reporting about a spiral notebook Mangione was carrying, which has given more insight into his alleged plans.

CNN said the notebook included “to-do lists” for a killing.

In one passage, Mangione concludes that using a bomb against his intended victim “could kill innocents”, reported CNN, citing a police source.

He wrote that shooting would be more targeted and mused about what could be better than “to kill the CEO at his own bean-counting conference”.

Meanwhile, Mangione’s arrest and release of his mugshot have triggered a “disturbing” online response from a growing fan base.

Online sellers looking to cash in on his infamy as a vigilante are selling merchandise referring to the suspect on T-shirts, hoodies, coffee mugs and shot glasses.

TMZ reports that Mangione’s online fan base is “next-level”, with the hype so wild that people are even offering to cover his legal bills.

Mangione’s lawyer Thomas Dickey said his office staff had fielded offers to cover Mangione’s fees.

After Mangione’s arrest at a McDonald’s in Pennsylvania, the state’s Governor Josh Shapiro raised concerns about his growing online celebrity.

“Some attention in this case, especially online, has been deeply disturbing, as some have looked to celebrate instead of condemning this killer,” Shapiro said.

Luigi Mangione’s mugshot sparked an online frenzy. Photo: Altoona PD

The release of Mangione’s mugshot, which gave the public its first look at the accused, sparked a frenzy of comments about his handsome looks.

Thompson’s murder also unleashed a wave of frustration from Americans struggling to afford medical care and those who have been denied claims or care.

The book Delay, Deny, Defend: Why insurance companies don’t pay claims and what you can do about it became a bestseller on Amazon in the week after the murder.

The extraordinary case has prompted White House press secretary Karine Jean-Pierre to stress that “violence to combat any sort of corporate greed is unacceptable”.

Mangione, who is from a wealthy Baltimore family, was an Ivy League graduate who was also the valedictorian of a private all-boys school in Maryland.

On Wednesday, he briefly struggled with officers and angrily shouted while being escorted into a Pennsylvania courthouse, as a clearer picture of his motives began to emerge.

Mangione, 26, turned toward journalists and yelled in part, “… completely out of touch and an insult to the intelligence of the American people!” before deputies pushed him away. It was not clear to what he was referring.

At the court hearing, his defence lawyer told a judge that Mangione would oppose extradition to New York, where he is charged with murder and other crimes.

That decision could delay the process by weeks but is unlikely to block his eventual transfer; for now, Mangione will remain in jail in Pennsylvania, where he faces gun and forgery charges.

Dickey said afterwards that Mangione planned to plead not guilty to the charges.

Thompson, the CEO of UnitedHealth’s insurance arm, one of the largest health insurers in the US, was shot dead on the morning of December 4 outside a hotel in Manhattan in what police said was a targeted attack, sparking a manhunt that led to Mangione’s arrest.

When arrested, Mangione was carrying a handwritten manifesto that offered insight into his mindset, according to police.

The New York Times reported that an internal New York City Police report analysing the document concluded that Mangione viewed the killing as a justified response to what he believed to be corruption in the healthcare industry.

“These parasites simply had it coming,” the manifesto said, according to the Times.

Mangione suffered from chronic back pain that limited his daily life, according to friends, social media postings and other news reports.

His profile on X shows a background image of an X-ray with what appears to be screws and plates inserted in a lower back.

From January-June 2022, Mangione lived at the Surfbreak co-living community in Honolulu, where he led a book club and surfed, hiked and rock-climbed, the founder of the group, RJ Martin, told Hawaiian outlet Civil Beat.

Martin said Mangione had suffered back pain caused by misaligned vertebrae pinching his spinal cord. He left for the mainland at some point for surgery.

But he went “radio silent” in June or July, Martin told Civil Beat.

Mangione never showed any indication of violence, Martin later told MSNBC.

“The Luigi that I knew is completely incompatible with an assassin,” he said, describing him as funny, kind and thoughtful.

At one point, Mangione suggested Surfbreak’s book club read the manifesto of Ted Kaczynski – the US domestic bomber known as the Unabomber – as a joke, according to Martin.

Mangione had a loaded ghost gun – an untraceable firearm assembled from parts – and a silencer, officials said after his arrest. Both the weapon and his clothing closely resembled those used by the gunman.

He also had multiple fake identifications, including a fraudulent New Jersey ID that matched the one used by the gunman to check into a Manhattan hostel days before the shooting, according to authorities.

Mangione’s family released a statement saying they knew only what had been reported in the media.

“Our family is shocked and devastated by Luigi’s arrest,” the family said in a statement posted to the X account of Maryland lawmaker Nino Mangione.

“We offer our prayers to the family of Brian Thompson and we ask people to pray for all involved.”

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