Law enforcement sources tell TMZ ... investigators found a spiral notebook they believe belongs to Mangione in which he wrote to-do lists of tasks that needed to be completed to kill Thompson, while explaining why these plans were necessary.

In one alarming passage, Mangione allegedly relished what it would be like "to kill the CEO at his own bean counting conference," believing that shooting his victim would be more targeted, rather than a bombing, which "could kill innocents."

Thoughts of using a bomb were apparently brought up when Mangione wrote about the infamous "Unabomber" Ted Kaczynski. As we reported, Luigi praised Kaczynski's manifesto as an "interesting read," but his friend RJ Martin tells us he didn't think Mangione was that moved by the material.

In addition to his spiral notebook, Mangione penned his own manifesto that railed against the healthcare industry. Our sources say Mangione wrote that the healthcare industry is full of "greed" and "these parasites had it coming," without naming Thompson.

Mangione is facing charges in Pennsylvania and in New York, where he's been charged with murder.