Luigi Mangione, the suspect in the killing of UnitedHealthcare CEO Brian Thompson, has expressed gratitude to those who have written to him while he remains jailed in a New York City detention center. A statement posted on a website run by his defense team marks his first public comments since he shouted to reporters before a Pennsylvania court appearance in December.

Luigi Mangione appears in Manhattan Supreme Court in New York on Dec. 23.

Mangione faces charges in New York, including first-degree murder in furtherance of terrorism, for Thompson’s Dec. 4 killing. Authorities say Thompson was shot from behind on a Manhattan sidewalk. Mangione was arrested days later in Pennsylvania, where he also faces weapons charges. Officials, including Manhattan District Attorney Alvin Bragg, have condemned the attack, calling it premeditated and intolerable. Pennsylvania Gov. Josh Shapiro also criticized those portraying Mangione as a hero.

Prosecutors allege Mangione used a ghost gun with a suppressor, and shell casings at the scene bore the words “deny,” “depose,” and “delay.” Authorities say he had writings detailing grievances with the health care system, though police confirmed neither he nor his family were insured by UnitedHealthcare. Mangione has pleaded not guilty to all charges.