On Dec. 13 last year, a stranger in a Brown University bathroom noticed a man dressed head to toe in black, wrong for the weather, mere hours before that same man opened fire on Brown University’s campus, killing two and wounding eight before vanishing. Investigators had almost nothing: no name, no plate, just a tip describing a gray Nissan with Florida plates. But that was enough: Providence police ran it through a network of more than 70 Flock Safety cameras, which reached back through 30 days of history, and found the car had passed 14 times since Dec. 1. “We...