Luigi Mangione Allegedly Considered Using Bomb to Murder CEO but Did Not Want to 'Kill Innocents': Source

Mangione was allegedly found with a notebook that contained a to-do list related to the murder, a senior law enforcement source says

Luigi Mangione, who authorities accuse of murdering UnitedHealthcare CEO Brian Thompson, allegedly wrote in a notebook that he considered bombing Manhattan to carry out the killing but did not want to "kill innocents," a senior law enforcement source tells PEOPLE.

The notebook, which police allegedly found on Mangione, contained a to-do list related to the killing, the law enforcement source says, and allegedly discussed different murder methods.

The source says Mangione, who is charged with murder by prosecutors in New York, allegedly preferred a targeted approach.

“What do you do? You wack the CEO at the annual parasitic bean-counter convention," Mangione allegedly wrote. "It’s targeted, precise, and doesn’t risk innocents.”

Mangione was taken into custody at a McDonald's in Altoona, Pa., on Monday, Dec. 9, days after a masked and hooded gunman fatally shot Thompson outside the New York Hilton Midtown in Manhattan as the CEO was headed to an annual investors event.

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Police allege that Mangione was found with a 3D-printed ghost gun, a silencer and fake IDs following his arrest. He was arraigned on five charges in Pennsylvania, including counts of forgery and weapons possession. He also faces potential extradition to New York, where he is charged with more serious offenses, including murder.

A law enforcement source previously told PEOPLE that Mangione was also found with a manifesto, in which he allegedly wrote that he acted alone but that the murder "had to be done."

"I do apologize for any strife or traumas, but it had to be done," Mangione allegedly wrote. "Frankly, these parasites had it coming."

Thompson, 50, who was laid to rest in a private setting, was a father to two sons.