In the federal criminal complaint -- filed Thursday -- prosecutors included several diary entries they claim Mangione made over the past few months ... all of which seem to point to Mangione as Thompson's killer.

An entry dated August 15 describes how he's "glad -- in a way -- that I've procrastinated" because it allowed him to learn more about a "Company-1" ... which is how the feds refer to UnitedHealthcare throughout the docs.

The big takeaway here ... this diary entry indicates Mangione allegedly cooked up this killing long before August -- hence his use of the word "procrastinated."

Also in the August 14 entry, prosecutors say Mangione wrote, "the target is insurance" because it "checks every box."

So, it doesn't seem like Mangione had a specific gripe just with the insurance industry, but corporate greed, in general ... and, targeting an insurance exec best served his purposes, symbolically.

The next diary entry included in the docs -- dated October 22 -- says "1.5 months. This investor conference is a true windfall ... and -- most importantly -- the message becomes self-evident."

About 6 weeks later, Thompson was killed in Manhattan as he entered the UHC investor conference.

According to feds, Mangione wrote that he planned to "wack" a CEO at the conference -- though the entry doesn't identify Thompson by name.

Additionally, federal prosecutors reference a letter recovered from Mangione during his arrest that was "addressed 'To The Feds'" ... they claim it states he was working alone -- and feds could verify it by checking relevant serial numbers to "verify this is all self-funded."

As we told you ... federal prosecutors filed the complaint Thursday -- on the same day Luigi was extradited from Pennsylvania back to NYC. These charges are tacked on top of the 3 state murder charges the Manhattan D.A.'s thrown at the Ivy League grad.