The altercation that left a moonlighting cop dead in New Orleans was sparked by a dispute over mask-wearing. WVUE reports that suspect John Shallerhorn got into an altercation with a staff member at a high school basketball game when he tried to enter without a mask on Friday night. Tulane Police Officer Martinus Mitchum, who was working security at the game, was escorting Shallerhorn out when then 35-year-old shot him twice, authorities said. Police say Shallerhorn had robbed someone in the parking lot minutes earlier.