Police have described receiving what a law enforcement source described to PEOPLE as a “manifesto” from Luigi Mangione, the man accused of murdering UnitedHealthcare CEO Brian Thompson. In the three-page document found on Mangione’s person, he allegedly claimed the murder “had to be committed.”

Mangione now faces murder charges as he remains incarcerated in Pennsylvania, where he was arrested after a six-day manhunt that began in New York City.

After Mangione was arrested at a McDonald’s in Altoona, Pennsylvania, authorities claimed they found a “ghost gun,” a silencer and fake IDs, as well as the manifesto.

A law enforcement source tells PEOPLE that in the manifesto, Mangione allegedly said: “To the Feds: I will be brief because I respect what you do for our country. To save you a lengthy investigation, I state clearly that I am not working with anyone.”

“I apologize for any disruption or trauma, but it had to be done,” Mangione reportedly continued. “Honestly, these parasites did it.”

Mangione was taken into custody after a McDonald’s customer told an employee that the 26-year-old, who was wearing a mask and looking at a computer at the fast-food restaurant, looked suspicious, a criminal complaint says. The employee then called the police.

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Thompson was killed outside New York’s Hilton Midtown Hotel in Manhattan as he went to an annual investor event.

Widely shared video footage of the Dec. 4 shooting appears to show a masked and hooded man firing multiple shots at Thompson.

Shell casings found at the crime scene reportedly had the words “deny,” “defend,” and “depose” written on them – a possible reference to “deny, delay, defend,” a phrase used in the insurance industry to describe a denial strategy claims is used.

Mangione also faces charges in Pennsylvania after he was allegedly found with a gun and showing a police officer a fake ID. He has not yet filed an appeal.