Accused killer Luigi Mangione appears to have been plotting his attack on UnitedHealthcare CEO Brian Thompson for at least three months — penning a desire to take out the head of a medical insurance company in a notebook laden with angry tirades against the industry, the feds revealed Thursday.

New details shining a light on the possible motive for the Dec. 4 ambush attack in Midtown were contained in a four-count federal complaint unsealed against Mangione, who was hauled back to the Big Apple to face murder charges.

The feds said Mangione — a 26-year-old University of Pennsylvania graduate who hails from a prominent Maryland family — allegedly had the notebook on him when Pennsylvania cops arrested him at a McDonald’s in Altoona five days after the shocking slaying.

“The Notebook contained several handwritten pages that express hostility towards the health insurance industry and wealthy executives in particular,” the complaint states.

In an entry dated Aug. 15, Mangione allegedly wrote “the details are finally coming together,” noting he “was glad” that he procrastinated in his preparations because it “allowed him to learn more” about UnitedHealthcare, the insurance behemoth Thompson led.

“The target is insurance,” he allegedly wrote, adding that UnitedHealthcare “checks every box.”

In October 22, he allegedly wrote “This investor conference is a true windfall,” apparently in reference to the company’s annual Investor Day conference at the Hilton in Midtown — outside of which he would allegedly later gun down Thompson, a 50-year-old married father of two.

He allegedly wrote of his intention to “wack” [sic] the CEO of one of the health insurance companies in attendance.

“Most importantly” what he ominously referred to as “the message” would become “self evident,” he wrote in the same entry, seemingly an oblique reference to his anti-insurer views and the aftermath the shooting would incite, according to court documents.

Mangione’s mom reported him missing to San Francisco police on Nov. 18 — just days before he would arrive in Manhattan via a Greyhound bus from Georgia, according to the feds.

After getting to town on Nov. 24, Mangione allegedly staked out the Hilton on Sixth Avenue, near West 54th Street, and “undertook extensive efforts to identify the victim, place the victim under surveillance, and track the victim’s whereabouts in the time period leading up to the murder,” the feds said.

When he was finally arrested, after a McDonald’s worker recognized him and called 911, investigators said Mangione also had on him a letter addressed “to the Feds” — in which he called hatching his alleged murderous scheme “trivial” and stated that he was working alone, the complaint states.

“I wasn’t working with anyone. This was fairly trivial: Some elementary social engineering, basic CAD, a lot of patience,” it said, referring to computer-aided design, according to the feds.

What we know about the shooting of UnitedHealthcare CEO Brian Thompson

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• Brian Thompson, the CEO of insurance giant UnitedHealthcare, was gunned down Wednesday outside a luxury Midtown hotel in a “brazen, targeted attack,” police said.
• Thompson was named CEO of UnitedHealth in April 2021. He joined the company in 2004. He was one of several senior executives at the company under investigation by the Department of Justice.
• Thompson’s wife, Paulette, said her husband had been getting threats before he was killed.
• Thompson’s shooting led to sick support online, and even spurred a tasteless lookalike competition in NYC.
• A person of interest has been nabbed by police officers inside a McDonald’s in Altoona, Pa.
• The suspect has been identified as Luigi Mangione, 26, originally from Towson, Md. He’s an Ivy League graduate who hated the medical community.

Follow along with The Post’s live updates on the news surrounding Brian Thompson’s murder.

The letter added: “P.S. you can check serial numbers to verify this is all self-funded.My own ATM withdrawals.”

Mangione made his initial appearance in a Manhattan courtroom Thursday on the federal charges, including murder through the use of a firearm, two counts of stalking and one count of possession of a firearm.

He also faces an 11-count indictment filed by state prosecutors, though it was not immediately clear when he would answer those charges in court.

“We are coordinating with our federal law enforcement partners on timing and logistics and will alert you when the defendant’s arraignment in New York Supreme Court is scheduled or there are further updates,” a rep for Manhattan District Attorney Alvin Bragg said in a statement.

In their own statement, a spokesperson for acting Manhattan US Attorney Edward Y. Kim said he “thanked the Manhattan District Attorney’s Office, which has brought a separate prosecution against Mangione, which is currently expected to proceed to trial before the federal case.”

Sources said federal prosecutors pursued their own case to ensure the alleged gunman will be eligible for the death penalty, which New York state did away with in 2004.

The accused shooter made a brief appearance in Pennsylvania court Thursday morning, where he waived extradition to New York, and was afterward flown to Long Island and then brought by helicopter to Lower Manhattan.

He arrived at Manhattan federal court — wrists and ankles shackled and clad in an orange prisoner jumpsuit — flanked by more than a dozen law enforcement officers, some carrying automatic rifles, with Mayor Eric Adams spotted walking behind him among the mass of cops.

A few people showed up to support the alleged cold-blooded killer, with one fan yelling out “thanks for representing him!” at Mangione’s defense attorney.

Mangione entered the courtroom a short time later in civilian clothes – a black quarter-zip sweater over a white shirt, tan khakis and orange prison shoes with a “12” emblazoned on his right foot. He was not wearing handcuffs, but the sound of ankle shackles being removed rang out in the courtroom.

Mangione sat calmly between his attorneys as magistrate Judge Katharine Parker read him his rights followed by the charges against him. He did not enter a plea during the hearing but replied “yes” when Parker asked if he understood his rights.

Karen Friedman Agnifilo — a former Manhattan state prosecutor leading Mangione’s defense — told the court she was blindsided by the new spate of federal charges, saying she’d never encountered anything like it in her career of “over three decades.”

She said she had initially been prepared to go to Manhattan Supreme Court for a hearing at 2 p.m. before Judge Gregory Carro.

“I don’t think they knew this was going to happen,” Agnifilo said, apparently in reference to prosecutors in the DA’s office.

“These seem like two different cases,” Agnifilo continued.

“Is there one case, two cases, two investigations? Is there a joint investigation? Frankly, I’ve never seen anything like that and what’s happening here.”

In a statement, Kim, the acting head of the powerful Southern District of New York, slammed Mangione, saying the killing was “a grossly misguided attempt to broadcast [his] views across the country.”

“But this wasn’t a debate, it was murder, and Mangione now faces federal charges,” he said.

Parker set Mangione’s next court appearance for Jan. 18.

Bragg, during an unrelated press conference Thursday, noted it wasn’t unprecedented for state and federal criminal cases to take place at the same time.

“This morning the defendant waived extradition in Pennsylvania and was picked up and is in the process of being transported by the NYPD here to New York,” the DA told reporters. “I’ll let the Southern District of New York speak for itself about their plans, but I do want to note that speaking generally, we’ve had state prosecutions and federal prosecutions proceed as parallel matters, and we are in conversations with our law enforcement partners.”

— Additional reporting by Ben Kochman