Luigi Mangione Went To Gun Range On Thailand Trip Months Before CEO Shooting

Luigi Mangione, the charged killer in the Brian Thompson murder case, took time from a vacation in Thailand to visit a shooting range ... a decision that now sticks out to some of his travel buddies.

Two German tourists who traveled extensively with Luigi in Asia in the months before the shooting look back on their interactions with Mangione in our new TMZ Investigates documentary, "Luigi Mangione: The Mind of a Killer."

Luigi's travel companions, Paul and Max, tell us Mangione hung back from a beach day in Thailand to go to a shooting range instead ... later reporting back to them that it was expensive. But, that's not the only strange thing Luigi did around the German tourists.

Paul and Max tell us Luigi was obsessed with a book from Indian author Jash Dholani, "Hit Reverse: New Ideas From Old Books" -- Dholani says Mangione attempted to buy 400 copies of his book, but the transaction was flagged and rejected by the bank. The author says Mangione was only able to buy 1 copy of his book.

The book may provide clues as to why Luigi became radicalized to the point where he allegedly gunned down a healthcare CEO ... but our documentary also explores Luigi's obsession with a second book about corporate greed ... which was his motive to kill.

What becomes clear in our doc ... healthcare was not Luigi's gripe ... he was more concerned with corporate greed ... and viewed the American healthcare system as a prime example.

Our upcoming documentary also explains why Luigi didn't have any particular experience with healthcare that led him to allegedly take out UnitedHealthcare's CEO ... rather, there were signs of mental illness with Luigi ... which raises an interesting defense at trial.