Two heart-shaped notes were hidden in the cardboard packaging of argyle socks delivered to Luigi Mangione ahead of his court appearance in Manhattan last month, according to prosecutors. “Know there are thousands of people wishing you luck,” one note said.

The episode was revealed in court documents made public on Wednesday as New York prosecutors and Mr. Mangione’s defense lawyers argued over his access to evidence and whether he was receiving special treatment.

Mr. Mangione was allowed to change his outfit for the court appearance, while “most incarcerated defendants must wear jail-issued clothing,” prosecutors wrote.

In spite of receiving the notes, “the defendant was permitted to wear the argyle socks,” prosecutors wrote, “which he first changed into and later changed out of because he felt that ‘they did not look good.’”

Mr. Mangione has been charged with killing Brian Thompson, the chief executive of UnitedHealthcare, on a sidewalk in front of a Midtown hotel on Dec. 4. Mr. Thompson was gunned down as he was arriving at an investor meeting.

Mr. Mangione was arrested five days later in a McDonald’s in Altoona, Pa., where the authorities said they found him with what they called a manifesto. The case has garnered international attention in the months since.