When an 80-year-old man at a bar near Buffalo, N.Y., noticed a fellow customer not wearing a face mask, he confronted him. The customer responded by swiftly pushing him to the ground, the police said. Five days later, the man was dead.

On Monday, the customer, Donald Lewinski, 65, of West Seneca, N.Y., was arrested and charged with criminally negligent homicide in the death of the 80-year-old, Rocco Sapienza. The case is believed to be one of the first of its kind in New York State.

Mr. Lewinski pleaded not guilty on Tuesday in court in Erie County. The charge carries up to four years in prison.

The encounter followed months of tension and often vitriolic confrontations across the country over masks.