Suicide Box (S13E16)

Alice Cushman (Claire Lautier), a beat cop in the 204, is shot as she exits a diner frequented by her colleagues. A fellow officer, Al Brennan (Chris Tardio), was at the scene (they had a brief extramarital fling) but no one knows who would want to kill Alice, a mother of a young child. Witnesses saw a blue car fleeing the scene, and when a nearby traffic accident arouses suspicion, they are able to get enough information from the BMW hit by the blue car (by analyzing the paint, which gives data on the type of car) and a partial identification from the license plate to find their vehicle. It's registered to Janet Thomas (Oni Faida Lampley), a nurse who claims her car was stolen. (iTunes)