[16] Police believe that he was inspired by "Unabomber" Ted Kaczynski's manifesto Industrial Society and Its Future (1995), and motivated by his own personal views on US health insurance.

[25][26] On social media, reactions to the killing included contempt and mockery toward Thompson and UnitedHealth Group, sympathy and praise for Mangione, and criticism of the American healthcare system and health insurance industry – primarily regarding claim denial practices.

[33] It and other insurers were named in an October 2024 report from the United States Senate Homeland Security Permanent Subcommittee on Investigations showing a surge in prior authorization denials for Medicare Advantage patients.

[40][41] Standing approximately 20 feet (6 m) away from Thompson when he arrived at the entrance, the assailant fired three times at him from a suppressed 9 mm pistol,[42][40][41] striking him in the back and right calf.

[43] In the closed-circuit television camera (CCTV) recording of the killing, the shooter manually cycles the gun action after each shot, leading observers to believe that his weapon was a malfunctioning semi-automatic pistol.

[74] In his bag they found a 3D-printed gun and a 3D-printed suppressor, which the police said are consistent with the weapon used in the shooting, and a falsified New Jersey driver's license with the same name as the one used by the alleged shooter to check into the Manhattan hostel.

[87] Mangione was charged in Blair County, Pennsylvania, with carrying a gun without a license, forgery, falsely identifying himself to the authorities, and possessing "instruments of crime" on December 9, 2024.

This is due to the fact that under New York law this kind of charge can only be levied against an individual if they “intended to intimidate or coerce a civilian population, influence the policies of a unit of government by intimidation or coercion and affect the conduct of a unit of government by murder, assassination or kidnapping.”[96][97] On December 19, Mangione was extradited to New York and transferred to the Metropolitan Detention Center in Brooklyn following his first appearance in the Manhattan federal court.

Agnifilo said that Mangione might not receive a fair trial due to publicity from law enforcement presentations of him, blaming Mayor of New York City Eric Adams for "unnecessary" high-profile perp walks.

[104][105][106][107] The document directly addresses federal law enforcement, states that the killing was a lone act and that methods involved "elementary social engineering, basic CAD, [and] a lot of patience."

It references a spiral notebook containing notes and to-do lists, mentioned that "[the shooter's] tech was locked down" due to his engineering work, and apologizes for causing "strife or traumas" while insisting that "it had to be done.

[17] Mangione's account posted a Goodreads review of Industrial Society and Its Future,[115] describing Kaczynski as "rightfully imprisoned" and was critical of his use of violence against innocent individuals.

"[116][117][118] A report on the killing by the New York City Police Department's Intelligence & Counterterrorism Bureau[119] was obtained by independent journalist Dan Boguslaw, and published in full by Klippenstein on December 26.

The report assessed that Mangione was motivated by "what he perceives as a 'parasitic' health insurance company and industry as a whole, as well as broader objections to corporate greed and a concern for modern society".

Steve Kopack, writing for NBC News, noted that "the downturns largely reflect investors' reactions to public outrage at the health care system in the wake of the killing".

[125][126][127][128] Social media users shared stories of health insurance claim denials online,[129][130][131] and joked about the killing with memes and gallows humor.

"[138] The Nation said that the case falls into the anarchist tradition of propaganda of the deed,[139] with The New Hampshire Gazette stating that these types of acts are "meant to show the broader public that, while the prevailing political and economic systems might have been powerful and omnipresent, they [are] not omnipotent.

"[164] UnitedHealth Group CEO Andrew Witty defended the company's claim denial practices in an internal video which was leaked after the fatal shooting of Thompson.

"[172][133] Michael Sherman, the former chief medical officer at Point32Health, justified the concerns of health insurance executives, saying, "It doesn't seem paranoid to worry that someone who's had services denied that they may believe are important might be in an emotionally unstable state.

"[132] In response to the killing, public officials including Minnesota governor Tim Walz and Senator Amy Klobuchar expressed dismay and offered condolences to the family.

[177] Outgoing Department of Homeland Security secretary Alejandro Mayorkas said that social media rhetoric about the killing was alarming, and that "It speaks of what is really bubbling here in this country, and unfortunately we see that manifested in violence, the domestic violent extremism that exists".

He added that the public reaction to Thompson's killing did not surprise him because "We waste hundreds of billions a year on health care administrative expenses that make insurance CEOs and wealthy stockholders incredibly rich while 85 million Americans go uninsured or underinsured".

[183] Democratic Senator Elizabeth Warren of Massachusetts said: "The visceral response from people across this country who feel cheated, ripped off, and threatened by the vile practices of their insurance companies should be a warning to everyone in the health care system.

[189] Zeynep Tufekci, a professor of sociology and public affairs at Princeton University and New York Times columnist, said that the public reaction to the fatal shooting of Thompson resembled the reaction to the very high levels of corporate greed, exploitation, and economic inequality during the American Gilded Age, a period characterized by violent "political movements that targeted corporate titans, politicians, judges and others".

[107][201][105][106] In an interview with Democracy Now!, Klippenstein blamed paternalistic attitudes in corporate media and the possibility of alienating law enforcement sources for the reluctance to publish the alleged manifesto's full text.

[208] Private security intelligence firm DragonFly assessed that further attacks on business leaders were unlikely, and that political groups that celebrate the murder "will almost certainly use non-violent – but still threatening – tactics" like "staging protests outside corporate offices, verbally confronting executives and swatting them.

The report assessed that "violent rhetoric and threats to healthcare and other corporate executives will increase" due to the shooting and recommended multiple personal security practices for "high-profile government and private sector individuals".

[215] In January 2025, a woman was arrested at the United States Capitol and charged with plotting to kill billionaire investor Scott Bessent on the day that he was confirmed as Trump's treasury secretary.

Anchors Colin Jost and Michael Che joked about the killing during the Weekend Update, saying "It really says something about America that a guy was murdered in cold blood and the two main reactions were 'Yeah, well healthcare stinks,' and also, 'Girl, that shooter hot'".

[219][220][221] On December 21, Saturday Night Live Weekend Update host Colin Jost mentioned Mangione during a joke, which was met with "rapturous applause" from the audience.