New York prosecutors have filed murder and other charges against Luigi Nicholas Mangione in the killing of UnitedHealthcare's CEO, according to an online court docket.

Mr Mangione remained jailed in Pennsylvania, where he was charged with possession of an unlicensed firearm, forgery and providing false identification to police.

Police arrested him on Monday after he was spotted in a Pennsylvania McDonald's by a customer who thought he looked like the suspected killer of UnitedHealthcare CEO Brian Thompson.

The 26-year-old was in possession of a gun, silencer, fake IDs, and a handwritten document suggesting he had "ill will towards corporate America", police said.

NYPD Chief of Detectives Joseph Kenny said Mr Mangione was born and raised in Maryland, had ties to San Francisco and a last known address in Honolulu, Hawaii.

"Our family is shocked and devastated by Luigi's arrest," Mr Mangione's family said in a statement posted on social media late on Monday, local time, by his cousin, Maryland state politician Nino Mangione.

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

On Monday, Mr Mangione was sitting in the rear of the McDonald's, wearing a blue medical mask and looking at a laptop computer, when he was arrested, court documents said.

A customer saw him and an employee called 911, said Kaz Daughtry, an NYPD deputy commissioner.

Altoona Police Officer Tyler Frye said he and his partner recognised the suspect immediately when he pulled down his mask.

"We just didn't think twice about it. We knew that was our guy," he said.

When one of the officers asked if he'd been to New York recently, he "became quiet and started to shake," according to a criminal complaint based on their accounts of the arrest.

In his backpack, police found a black, 3D-printed pistol and a 3D-printed black silencer, the complaint said.

The pistol had a metal slide and plastic handle with a metal threaded barrel. He was taken into custody just after 9 am, local time, police said.

Mr Mangione had clothing and a mask similar to those worn by the shooter and a fraudulent New Jersey ID matching one the suspect used to check into a New York City hostel before the shooting, NYPD Commissioner Jessica Tisch said.

The 50-year-old victim, Mr Thompson, was shot in the street as he walked to the Hilton hotel in Midtown Manhattan for an investor conference.

It was widely reported that the bullet casings found at the scene had the words "delay", "deny" and "depose" written on them.

It was interpreted as a possible reference to "delay, deny, defend", a phrase often used by critics of America's corporate health insurance industry.

The sector has a reputation for underpaying and refusing to pay claims, and medical debt is now the most common cause of personal bankruptcy in the US.