By Dalia Faheid, CNN

(CNN) — Diary entries written by Luigi Mangione reveal the now 27-year-old’s detailed thinking before the killing of UnitedHealthcare CEO Brian Thompson last year, a new court filing shows.

A red notebook was recovered by police at the time of his December 9 arrest at a McDonald’s in Altoona, Pennsylvania. In diary writings contained in that notebook, Mangione vents about his frustrations with the health insurance industry and his intent to carry out an attack. The entries also shed light on Mangione’s focus on the court of public opinion and how he intended to gain widespread support through the alleged killing.

The new filing from the Manhattan District Attorney’s Office is a response to a motion filed last month by Mangione’s defense team seeking to stay or dismiss the New York case against him. Mangione has pleaded not guilty to all federal and state charges against him.

In August 2024, roughly four months before he allegedly shot and killed Thompson in midtown Manhattan, Mangione wrote in his diary: “I finally feel confident about what I will do. The details are coming together. And I don’t feel any doubt about whether it’s right/justified. I’m glad-in a way-that I’ve procrastinated bc it allowed me to learn more about [UnitedHealthcare].”

“The target is insurance. It checks every box,” he continued in the August 15 entry.

That summer, Mangione – who had an active social media presence for years – appeared to stop posting online, prompting worried messages from some of his friends.

In October, another diary entry reads, “1.5 months. The investor conference is a true windfall. It embodies everything wrong with our health system, and-most importantly-the message becomes self-evident. The problem with most revolutionary acts is that the message is lost on normies.” He then goes on to explain his reasoning for not targeting the health care industry through a bombing, writing that “innocent” lives would be unaffected by his attack.

At the time of that writing, Mangione – the privileged scion of a well-to-do family, high school valedictorian and Ivy League graduate – had reportedly vanished from view of his loved ones.

“Nobody has heard from you in months, and apparently your family is looking for you,” one user posted on X in October, tagging an account belonging to Mangione. “I don’t know if you are okay,” another posted.

Mangione allegedly gunned down Thompson on a busy sidewalk as Thompson walked toward a Manhattan hotel hosting his company’s investors’ conference, authorities said. The suspect appeared to be driven by anger against the health insurance industry and against “corporate greed” as a whole, according to an NYPD intelligence report obtained by CNN.

The previously unreported writings lay bare Mangione’s plans to target the insurance industry. Prosecutors argue in the filing that his actions motivated a barrage of threats against health insurance workers and made them worry for their safety.

CNN has reached out to Mangione’s attorneys for comment.

In the months since the fatal shooting of Thompson, Mangione has become a cult-like figure. There has been a massive outpouring of support on social media and at his court appearances from people with deep frustration and anger at the American for-profit health care system. They see the American health insurance industry as broken, overly expensive and quick to deny coverage.

The majority of insured US adults had at least one issue with their health insurance within the span of a year, including denial of claims, according to a survey released in June 2023 by nonprofit health policy research group KFF. A legal defense fund in support of Mangione has raised more than $1 million as of Wednesday.

“Bombs=terrorism,” Mangione writes

In the diary entries, Mangione – who allegedly used a ghost gun to carry out the killing of Thompson – criticizes bombers: “They commit an atrocity whose horror either outweighs the impact of their message, or whose distance from their message prevents normies from connecting the dots.”

In the October 22 entry, Mangione went on to ask, “Do you bomb the HQ? No. Bombs=terrorism. Such actions appear the unjustified anger of someone who simply got sick/had bad luck and took their frustration out on the insurance industry, while recklessly endangering countless employees.”

Instead of carrying out a bombing, targeting the CEO at the conference is “targeted, precise and doesn’t risk innocents,” adding that it would bring light to the event and the “greed” of its attendees.

He also appears to reference the Unabomber, Ted Kaczynski, calling such attacks “counter-productive” because they would lose public support.

“Normies categorize him as an insane serial killer, focus on the act/atrocities themselves, and dismiss his ideas,” Mangione writes. “And most importantly—- by committing indiscriminate atrocities he becomes a monster, which makes his ideas those of a monster, no matter how true. He crosses the line from revolutionary anarchist to terrorist-the worst thing a person can be.”

The killing sent waves of fear through UHC

Mangione’s intentions to target the insurance industry were made clear by the writings, which prosecutors say prove that he committed first-degree murder in furtherance of terrorism, the Wednesday filing states. UnitedHealthcare became a symbol of the health insurance industry that Mangione aimed to abolish, prosecutors said.

He was not insured by UnitedHealthcare from 2014 to 2024, prosecutors say, but at the time of his arrest, Mangione allegedly had a handwritten notebook that expressed “hostility toward the health insurance industry and wealthy executives in particular,” according to a federal complaint.

“Having no business relationship with them, he chose UHC solely because they were the largest health insurance company and one of the country’s largest companies by market cap,” the new filing said.

Notably, three 9mm shell casings from the crime scene had the words “delay,” “deny” and “depose” written on them, the NYPD has said, an apparent nod to a 2010 book critiquing insurance industry tactics.

Mangione hoped the alleged killing would intimidate health insurance employees, cause the public to focus on greed in the health insurance industry and prevent investors and financial analysts from investing in the industry, according to the filing.

The killing of the husband and father of two struck fear in C-suites across the country, as an NYPD intelligence report obtained by CNN warned online rhetoric could “signal an elevated threat facing executives in the near-term.”

Mangione inspired some individuals to partake in a “broader campaign of threats of violence” against UnitedHealthcare employees and other health insurance workers, the document says.

In the aftermath of Thompson’s killing, threats had been aimed at other UnitedHealthcare executives and employees also reported feeling unsafe, prosecutors said.

UnitedHealthcare doctors sending out denial letters to customers feared for their safety, requesting that they not be required to sign their names to the letters, prosecutors said. Some physicians quit their jobs out of fear of retribution.

The company advised employees not to wear company branded clothes, and online threats prompted it to pull pictures of senior executives from its website, the filing states.

Meanwhile, the company’s call center received a slew of death threats, the filing said. “You are gonna hang,” one caller said. “That means that the killing of Brian Thompson was just a start. There are a lot more that are gonna be taken out. The only question is whether you’re gonna be their collateral damage when its done or not.”

Police were hired to protect the company’s headquarters in Minnesota. Threats were also made to employees at the company’s New York City office.

Forty company executives received personal security, with one executive who received threats dying her hair and moving into another home out of fear for her safety, according to the filing.

Other health insurance executives, including Emblem Health’s CEO, were also targeted. Posters were put up outside the company’s headquarters with the CEO’s picture that read “Health Care CEOs should not feel safe. Deny, Defend Depose.”

The killing “demonstrated his concerted effort to broadcast his message of ideological intimidation as broadly and loudly as possible,” prosecutors said.

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CNN’s Kara Scannell, Zoe Sottile, Lauren del Valle and Michelle Watson contributed to this report.