Police said Mangione had with him a 9mm handgun that matched the one used in the shooting and other items including a fake ID and a notebook described by authorities as a "manifesto" in which they say he expressed hostility toward the health insurance industry and wealthy executives.
Among the entries in the notebook, prosecutors said, was one from August 2024 that said "the target is insurance" because "it checks every box" and one from October that describes an intent to "wack" an insurance company CEO.
UnitedHealthcare is the largest health insurer in the U.S., though the company said Mangione was never a client.
Mangione's lawyer, Karen Friedman-Agnifilo, has said she would seek to suppress some of the evidence seized during his arrest. She has also taken issue with the parallel prosecutions, accusing "warring jurisdictions" of turning Mangione into a "human ping-pong ball."