UnitedHealthcare CEO slaying suspect Luigi Mangione's first meal behind bars revealed

ALTOONA, Pa. – When Pennsylvania police arrested a former Ivy League computer scientist in connection with the New York City assassination of a health insurance CEO in New York City, he was shaking in his chair at an Altoona McDonald's, according to court documents.

By the time he had his first meal behind bars, a square slice of pizza that officers shared with him and others at the police station, he'd calmed down, a law enforcement source told Fox News Digital.

"While he was nervous and shaking during the arrest – when he was in the jail cell his demeanor was relatively calm," the source said. "He didn't appear angry or scared."

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Luigi Mangione, 26, was arrested peacefully, according to authorities, but he declined to speak with investigators. As NYPD brass and Manhattan prosecutors were driving in at speeds above 80 mph from nearly 300 miles away, the local cops received donations of food and coffee from supporters around the country after the news broke. They shared some of it with the suspect.

"We pride ourselves on our hospitality, whether our guests are there willingly or not," the source said. 

Police found Mangione on Monday morning after a worker and customer at the McDonald's spotted him snacking on a breakfast meal and called police.

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Responding officers, including a rookie cop who received praise from leaders in New York and Pennsylvania, immediately recognized Mangione as the suspect wanted in connection with the New York City ambush shooting of UnitedHealthcare CEO Brian Thompson, authorities said.

They found Mangione wearing a beanie and a coronavirus mask, sitting with a laptop at a table in the fast-food joint. 

WATCH: McDonald's customer recognized suspect's backpack, jacket

Prosecutors alleged in court that he had the suspected murder weapon, a so-called ghost gun with 3D-printed parts and a suppressor, the same fake ID used to check into a Manhattan hostel before the shooting, $10,000 in American and foreign cash, and a "Faraday bag" used to block cell service.

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Mangione told the judge that the cash wasn't his.

"I don't know where that money came from," he said. "It must have been planted. I don't have that kind of cash."

The bag, he added, was just a waterproof bag. An online search found several companies that sell Faraday containers describe them as also being waterproof.

Surveillance video taken outside a Midtown Manhattan Hilton hotel shows a masked assassin sneak up behind Thompson on the sidewalk around 6:45 a.m. last Wednesday, Dec. 4. 

Thompson was on his way to a shareholder conference at the venue set to begin later that morning when the gunman opened fire from behind.

As the CEO collapsed on the street, a woman who witnessed the attack fled in one direction, and the masked figure casually walked off in the other. Police tracked his movements throughout New York City to a bus depot, where he left about an hour after the slaying.

WATCH: Luigi Mangione delivers 2016 valedictorian speech

Surveillance images taken from a hostel he stayed at near Central Park circulated widely online as police launched an interstate manhunt for the suspect.

Mangione is facing a slew of charges in New York in connection with the murder, as well as additional charges, including unlawful possession of a firearm and a forged ID, in Pennsylvania.

He graduated with bachelor and master's degrees in computer science from the University of Pennsylvania in 2020 and comes from a prominent Baltimore family.

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He also attended the Gilman School, a private prep school in the city, where he was valedictorian in 2016.

He was a periodic poster on Goodreads, the literature-focused social media site, where he wrote a review for a book by Unabomber Ted Kaczynski. 

Writing about Kaczynski's "Industrial Society and Its Future," he quoted another online "take that [he] found interesting."

"When all other forms of communication fail, violence is necessary to survive," he wrote. "You may not like his methods, but to see things from his perspective, it's not terrorism, it's war and revolution."

NYPD Chief of Detectives Joseph Kenny told Fox News over the weekend that it was too soon to allege a motive but acknowledged that the suspect did leave potential clues behind.

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At the scene, police found bullet casings with handwritten words on them, "depose," "deny" and "defend," drawing comparisons to the book "Delay, Deny, Defend: Why Insurance Companies Don't Pay Claims and What You Can Do About It" – and speculation swirled that the slaying may have grown out of resentment for a denied claim.

The book was not found on Mangione's Goodreads account when accessed before it was set to private Monday.

Fox News' David Hammelburg contributed to this report.