On the evening of 29 December 2011, the police officer Clifton Lewis was moonlighting as a security guard at a Chicago minimart when two men walked in. They shot Lewis several times, then took off with his gun and police star. A week later, officers had their suspects: four men affiliated with a gang called the Spanish Cobras. For hours, under intense police questioning, they all said they didn’t do it. That didn’t seem to matter. This is episode one of Off Duty, an investigation by the Guardian’s Melissa Segura