The process began with a quick review of the transit agency’s surveillance cameras, which captured the alleged gunman shooting the victims execution style. Law enforcement followed the suspect, through real-time footage, across the rapid-­transit system. Police officials circulated the images to transit staff and to thousands of officers. An officer in the adjacent suburb of Riverdale recognized the suspect from a previous arrest. By the time he was captured at another train station, just 90 minutes after the shooting, authorities already had his name, address, and previous arrest history.