ALTOONA, Pa. — While UnitedHealthcare CEO Brian Thompson's alleged killer's motive has not been released by officials, the public has been speculating that the suspect had strong grievances with both the health care industry and capitalism in general.

Authorities arrested Luigi Mangione, 26, in a McDonald's in Altoona, Pennsylvania, on Monday, at which point he allegedly presented local police with a fake ID and appeared to start shaking when they asked if he had been in New York recently. 

Authorities also found a handwritten manifesto condemning the health care industry, as NYPD Chief of Detectives Joseph Kenny previously told Fox News. The manifesto specifically mentioned UnitedHealthcare.

Curtis Sliwa, founder of New York City's Guardian Angels, told Fox News Digital that "in every era, there are people who go out, take the law into their own hands, and they become heroes."

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"In this case, I think what is most stunning to me is the number of women who have embraced Luigi not because he is a good-looking guy with abs, but rather because of what they believe is this tremendous overreach, especially if this particular health care company in which you have so many denials of procedures that had been guaranteed," Sliwa said.

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He added his belief that the anger regarding the health care industry that has surfaced from Thompson's murder is misplaced.

The Altoona Police Department in a Tuesday press conference said it is actively investigating threats toward civilians and McDonald's employees after reports surfaced that a worker at the fast-food chain called 911 to report a sighting of Mangione, who was arrested shortly after the call. The police department has also been receiving threats.

"This is not the way we handle things," Sliwa said. "You don't start threatening the person who was responsible for ID'ing him. That's sort of like the code of gangs when they say ‘snitches get stitches and end up in ditches.’"

Nicholas Creel, associate professor of Business Law and Ethics at Georgia College & State University, told Fox News Digital that "America is currently in a period of rising populist sentiment, one where anger at elites, particularly wealthy corporate executives, is increasingly common."

"This explains why so many people across social media began expressing support for Luigi Mangione’s actions as soon as they learned who his victim was," Creel said. "A health insurance CEO is perhaps the pinnacle of a wealthy elite that many now see as being responsible for their financial troubles, so it is anything but surprising to see so many people identifying more with the criminal than the victim in this case."

Creel added that rhetoric from those supporting Mangione can "absolutely have a clear and negative effect on the safety of wealthy corporate executives, as it gives people a specific target for their anger."

"This is why so many other health insurance companies immediately took down identifying information for their executive teams, to try to make it at least a little more difficult for them to be doxxed by the populist mobs that see this murder as a rallying point to strike out against corporate elites," Creel said.

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Dr. Rachel D. Miller, LMFT, founder of Hold the Vision Therapy in Chicago, told Fox News Digital that she is seeing the impacts of capitalism inequality in her practice and with her clients every day.

"What I have been seeing in my clinical work is increased anxiety around employment stability, increased challenges in the ability to make ends meet and an underlying fear of any catastrophic event, such as loss of vehicle, loss of health care," Miller said.

Bill Knack, founder and president of First Responder Protective Services, which delivers executive protection for high-profile leaders and dignitaries across the country, similarly told Fox News Digital that this rhetoric "leads to a slow burn of anger that turns into action."

"It pushes people who already resent wealth into following messages that single out certain leaders," he said. "We have managed cases where executives faced direct, specific threats after their names spread through similar online writings."

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Mangione, a private-high-school and Ivy League-educated young professional from the Baltimore area, is charged with multiple counts in both Pennsylvania and New York, where he is facing a murder charge. 

Since his arrest, internet sleuths have been digging through Mangione's vast and documented social media presence, with many users praising the murder suspect for allegedly killing Thompson, who was a married father of two originally from a small town in Iowa.

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Even a professor at Mangione's alma mater, the University of Pennsylvania, took to TikTok and Instagram to voice praise for the suspect that she has since retracted.

UPenn School of Arts and Sciences Deputy Dean Jeffrey Kallberg issued a statement on Wednesday regarding the post from UPenn Assistant English Professor Julia Alekseyeva.

"Much concern was raised by recent social media posts attributed to Assistant Professor Julia Alekseyeva," Kallberg said. "Her comments regarding the shooting of Brian Thompson in New York City were antithetical to the values of both the School of Arts and Sciences and the University of Pennsylvania, and they were not condoned by the School or the University. Upon reflection, Assistant Professor Alekseyeva has concurred that the comments were insensitive and inappropriate and has retracted them."

Former Washington Post and New York Times reporter Taylor Lorenz told Piers Morgan that she and "so many other Americans" felt "joy" upon hearing the news of Thompson's death.

"I do believe in the sanctity of life and I think that’s why I felt, along with so many other Americans, joy, unfortunately," Lorenz told Morgan on Monday, adding later, "Maybe not joy, but certainly not empathy."

On Goodreads, a platform where users review and keep track of books they've read, a profile matching Mangione wrote a review for a book by the Unabomber, Ted Kaczynski. 

"It's easy to quickly and thoughtless write this off as the manifesto of a lunatic, in order to avoid facing some of the uncomfortable problems it identifies," he wrote. "But it's simply impossible to ignore how prescient many of his predictions about modern society turned out."

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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."

Mangione recently lived in a Honolulu-based coliving space called Surfbreak Coliving, which is described on its website as a "co-working space for remote workers and digital nomads."

"The entire time that he [Mangione] lived at Surfbreak, he was a great community member," his former roommate, R.J. Martin, told Fox News' Jesse Waters on Tuesday. "He kind of followed by our values and our ethics to leave things better than you found them. He was always contributing, taking care of other people. And even after he moved out, he came and hung out and contributed to the community, spearheaded a book club."

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In New York, Mangione faces one count of murder, two counts of second-degree criminal weapons possession, one count of second-degree possession of a forged document and one count of third-degree criminal weapons possession.

In Pennsylvania, he faces one count of forgery, one count of carrying a firearm without a license, one count of tampering with records or identification, one count of possession of instruments of a crime and one count of presenting false ID to law enforcement, according to court documents.

Fox News' Mollie Markowitz contributed to this report.