Accused murderer Luigi Mangione was carrying a notebook that outlined plans to kill a CEO when he was arrested at a McDonald's restaurant earlier this week, according to US media reports.
Mr Mangione, 26, remains in custody in Pennsylvania, where his lawyer is fighting attempts to extradite him to New York to face a murder charge.
He's accused of shooting Brian Thompson, the CEO of insurer UnitedHealthcare, just before an investor conference in Manhattan a week ago. His lawyer has indicated he will plead not guilty to all charges brought against him.
Mr Mangione's notebook was accompanied by a written "manifesto" railing against corporate America. Law enforcement sources told CNN and the New York Times the notebook contained the words:
"What do you do? You wack the CEO at the annual parasitic bean-counter convention. It's targeted, precise, and doesn't risk innocents."
New York police are continuing to build a case against Mr Mangione.
On Wednesday, local time, police commissioner Jessica Tisch said bullet casings found at the crime scene matched the gun he was carrying when he was arrested.
His fingerprints also matched those on a water bottle and a snack bar wrapper found nearby.
Earlier, investigators confirmed Mr Mangione had written about his struggles with a back injury, which they believe he suffered on July 4 last year.
"Some of the writings that he had, he was discussing the difficulty of sustaining that injury," New York Chief of Detectives Joseph Kenny told Fox News.
"So we're looking into whether or not the insurance industry either denied a claim from him, or didn't help him out to the fullest extent."
Friends have told American news outlets Mr Mangione suffered the debilitating injury while surfing in Hawaii.
"I know it was really traumatic and difficult," said friend RJ Martin, who knew Mr Mangione when he was living in Honolulu. "When you're in your early twenties and you can't do some basic things, it can be really, really difficult."
American authorities have been grappling with unusually high levels of public support for Mr Mangione, a high-achieving computer scientist from a prominent Maryland family.
The shooting sparked a flood of social media content criticising corporate healthcare and, later, hailing Mr Mangione as a hero.
It prompted Pennsylvania Governor Josh Shapiro to warn Americans against celebrating "vigilante justice". "Hear me on this, he is no hero," he told a media conference after the arrest.
Mr Mangione has been charged with murder in the second degree in New York, as well as several weapons offences in both New York and Pennsylvania.
