• A notebook found with Luigi Mangione contains a description of killing a CEO that matches details of his alleged murder of UnitedHealthcare CEO Brian Thompson in New York last week.
• The notebook was discovered when the Ivy League graduate was arrested at a McDonald's in Altoona, Pennsylvania, by police who found a gun, a silencer and ammunition in his backpack.
• The gun matches shell casings from the shooting outside the Hilton hotel and Mangione's fingerprints match those on a water bottle and snack bar found near the scene, police said.

A notebook found with Luigi Mangione contains a description of killing a CEO that matches details of his alleged murder of UnitedHealthcare's CEO in New York last week, NBC News reported Wednesday.

"What do you do?" a section of the notebook says.

"You wack the CEO at the annual parasitic bean-counter convention," the notebook says. "It's targeted, precise and doesn't risk innocents."

The notebook was discovered when the Ivy League graduate was arrested Monday in Altoona, Pennsylvania, by police who found a gun, a silencer and ammunition in a backpack he had with him at a local McDonald's.

That gun matched three shell casings found outside the Hilton hotel in midtown Manhattan where UnitedHealthcare CEO Brian Thompson was fatally shot on Dec. 4, New York police said Wednesday.

New York Police Commissioner Jessica Tisch also revealed that fingerprints taken from Mangione match prints on a water bottle and a Kind snack bar left near the shooting scene. Police last week had said that the then-unidentified gunman who killed Thompson purchased water and protein bars before the shooting.

Thompson, a father of two, was scheduled to speak Dec. 4 at an investor day hosted by UnitedHealth Group, his company's parent, which was being held at the Hilton.

Surveillance video of Thompson's killing shows a masked man firing a gun that appears to be attached to a silencer at the CEO from behind, just outside the Hilton, as another person stands nearby.

Hours after Thompson's slaying, Tisch told reporters, "I want to be clear: At this time, every indication is that this was a premeditated, preplanned, targeted attack."

It was previously reported that Mangione also had a handwritten note with him that read, in part, "These parasites simply had it coming," when he was arrested in Altoona.

The note, which said he was not "working with anyone," also said, "I do apologize for any strife or traumas but it had to be done."

The note criticized UnitedHealthcare, the U.S. health-care industry and corporations.

Mangione, who holds two degrees from the University of Pennsylvania, reportedly has suffered from significant back pain for years.

UnitedHealthcare, which is the largest private payer of health insurance benefits in the U.S., has been criticized for denying claims of customers.

Mangione is being held without bail in Pennsylvania on state firearms and forgery charges related to his arrest. The forgery charge relates to his alleged possession of several false identification documents, including one that was allegedly used to check into a hostel on Manhattan's Upper West Side nearly two weeks before Thompson's slaying.

He is charged with murder and gun crimes in Manhattan.

At a Blair County Court hearing on Tuesday, Mangione refused to waive extradition to New York to face charges there for Thompson's killing.

The Manhattan District Attorney's Office and New York Gov. Kathy Hochul said they will file paperwork required for a judge in Pennsylvania to decide whether to order Mangione's extradition.

— WNBC's Jonathan Dienst contributed reporting.