Luigi Mangione, in the shooting death of United Healthcare CEO Brian Thompson, once belonged to a group of Ivy League gamers who played assassins, a member of the group told NBC News.

In the game, called "Among Us," some players are secretly assigned to be killers in space who perform other tasks while trying to avoid suspicion from other players.

Alejandro Romero, who attended the University of Pennsylvania with Mangione and was a member of the same Discord group, said he was shocked when news broke on social media that Mangione had been taken into police custody.

"I just found it extremely ironic that, you know, we were in this game and there could actually be a true killer among us," he said.

"As soon as his photo and name popped up on X, my friend texted me asking if I knew him, and then either I was calling some 10 friends or they were calling me," Romero added. "I didn't speak to anybody today who wasn't already aware of what had happened."

"Among Us" is a child-friendly whodunnit game in which a group of friends run around several space station-like maps. The “crewmates,” who are simply drawn, cartoonish-looking astronauts, run around the ship and complete tasks.

The intent of the game? One member is a killer and attempts to off the other crewmates. The death animations are silly and shown as someone getting stabbed in the back or having their neck snapped. Once a body is discovered, a crewmate reports it and a conversation ensues to deduce who the gamers think committed the crime. If they’re wrong, an innocent player will be ejected in a humorous animation of them floating off into space. If they’re right, the crewmates win. If the “killer” gets all the crewmates or gets all the innocent ones ejected, they win.

The game has been wildly popular, especially during the pandemic, and particularly among young children because of its simple mechanics, colorful cartoonish nature, and unpredictability. The game is so beloved by younger gamers, dressing as a “crewmate” is often a popular children’s costume at Halloween

Mangione, 26, was arrested Monday morning in a McDonald's restaurant in Altoona, Pennsylvania, after an employee spotted him.

Police found a firearm, believed to have been 3D-printed, and a handwritten document on Mangione "that speaks to both his motivation and mindset," New York Police Commissioner Jessica Tisch said at a news conference.

He was also carrying fake identification and a passport, authorities said.

In New York, Mangione was charged with murder, possession of a loaded firearm, possession of a forged instrument and criminal possession of a weapon, according to court documents.

Authorities in Pennsylvania charged Mangione with carrying firearms without a license, forgery, tampering with records or identification, possessing instruments of crime and providing false identification to police.

In a statement on X on Monday night, a member of the Mangione family said they are "shocked and devastated by Luigi's arrest."

"We offer our prayers to the family of Brian Thompson and we ask people to pray for all involved," wrote Nino Mangione, a Republican member of the Maryland House of Delegates.

Romero, who said he has not spoken to or seen Mangione since 2020, described him as a typical college student who did not stand out to him.

"He just fit a mold," Romero said. "He just seemed like any other normal frat dude that you could see at a frat party."

His final year in college was cut short when the pandemic hit. Students were forced off campus in their last semester and did not return for commencement.

The Discord group was one way to stay connected, Romero said, but members began to go their separate ways as they got full-time jobs or embarked on long trips.

During some of those years, Mangione left behind a digital footprint that included reviewing “Industrial Society and Its Future,” also known as the “Unabomber Manifesto” by Ted Kaczynski, on Goodreads, a platform for book reviews and recommendations. It served as the ideological reasoning for Kaczynski’s yearslong mail bomb campaign that killed three people and injured 23 others.

Mangione became significantly more active on X in 2021 after five years not posting or reposting content, according to a review of his account. 

Asked about the change in Mangione's online persona, Romero said that question is circulating among his friend group.

"I feel like people are unsure how to label him," he said. "I'm personally struggling to understand how this all fits."