New York, New York - The man accused of gunning down a health insurance executive in New York was found with a handwritten manifesto laying out complaints against the industry, police said Tuesday, giving a possible motive for the first time.

Luigi Mangione (26) is accused of murdering UnitedHealthcare chief executive Brian Thompson on the streets of Manhattan last week, triggering a nationwide manhunt.

He was caught on Monday after he aroused the suspicions of staff in a McDonald's restaurant in Altoona, Pennsylvania, where he fled following the brazen attack.

"I had an opportunity to read the manifesto," the New York Police Department's Chief of Detectives, Joseph Kenny, said on Good Morning America.

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"It's handwritten. He does make some indication that he's frustrated with the healthcare system in the United States."

Kenny said Mangione decried how the US health care system is among the most expensive in the world and yet the country has a lower life expectancy than other developed nations.

"He was writing a lot about his disdain for corporate American and, in particular, the health care industry," said Kenny.

Police have not confirmed reports the words "delay" and "deny" – language used by insurers to reject claims – were written on casings found at the scene.

Investigators have continued to interrogate Mangione over last week's murder, which triggered global headlines and sent shockwaves through the American business sector.

He appeared at a Pennsylvania court wearing a dark sweatshirt late Monday and was led inside by Altoona police flanked by NYPD detectives.

Mangione was later charged in New York on suspicion of one count of murder, two counts of second-degree criminal possession of a weapon, and other offenses.

He is next due to appear in court on December 23, and has yet to enter a plea.

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The suspect was apprehended by officers following a tip from staff at the McDonald's, where he was found wearing a mask and a beanie while using a laptop, and gave officers a fake ID, charging documents show.

They then searched him and found what police called a "ghost gun" capable of firing 9mm rounds and equipped with a suppressor that could have been made on a 3D printer.

When officers asked if he had been to New York recently, Mangione "became quiet and started to shake," according to the criminal complaint.

One of the fake IDs found was one used to check in to a Manhattan hostel ahead of the attack, police said.

Mangione appeared to have health issues, living with serious back pain and undergoing surgery for the condition last year, according to a New York Times report quoting friends.

A photo on what appeared to be one of his social media accounts featured an X-ray of a spine with a medical implant.