On a mild spring night in Chicago, a woman told her 18-year-old boyfriend she wanted money for a barbecue. He rounded up three teenage friends, each with a long criminal record, and, according to prosecutors, they donned masks, carried guns, and robbed four people, tossing two to the ground. They went searching for more victims in a stolen Kia; shortly after 1:30 a.m. they crossed paths with Aréanah Preston.

Preston, a police officer, had finished her shift and, still in uniform, parked across the street from her family home on the South Side. The 24-year-old was to receive a master’s degree in law the following week. The police department viewed her as a future leader; the FBI had talked with her about a job. The young men in the Kia saw her as a target. They ran at her; a grainy securit

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