(CNN) — Diary entries written by Luigi Mangione reveal the now 27-year-old’s detailed thinking before the killing of UnitedHealthcare CEO Brian Thompson last year, a new court filing shows.

A red notebook was recovered by police at the time of his December 9 arrest at a McDonald’s in Altoona, Pennsylvania. In diary writings contained in that notebook, Mangione vents about his frustrations with the health insurance industry and his intent to carry out an attack. The entries also shed light on Mangione’s focus on the court of public opinion and how he intended to gain widespread support through the alleged killing.

CNN’s Kara Scannell, Zoe Sottile, Lauren del Valle and Michelle Watson contributed to this report.

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