It's fiction. While it might seem like it could be based on true stories because of how gritty and real - world the police situations are presented, the storylines and characters are fictional. The showrunners likely took inspiration from general knowledge of police work and issues like corruption, but they've created their own unique story that isn't based on a particular true event.
It's made up. There's no real - life event that directly correlates to what's shown in the show.