Late in the afternoon on St. Patrick’s Day, Glen Grays, a 27-year-old African-American mail carrier, was making his rounds in Crown Heights, in Brooklyn, about to leave a package at 999 President Street. Mr. Grays prides himself on getting to know the community he serves, he told me on Wednesday. He figures out who is sick, or old, or enfeebled, and makes sure that their parcels, especially if they contain medication — “I can shake a box and usually figure that out,” he said — land directly at the doors of the people waiting for them, even if they live in fourth- or fifth-floor apartments, in walk-up buildings.

On this afternoon, Mr. Grays was descending the steps of his mail truck backward, as postal workers often do to minimize wear and tear on the knees, when out of the corner of his eye he noticed a car making a sharp right turn onto President from Franklin Avenue. Mr. Grays shouted at the driver, climbing back up the steps to avoid getting sideswiped. The black car, in Mr. Grays’s telling, came tearing back his way in reverse. The driver said to him, Mr. Grays recounted, “I have the right of way because I’m law enforcement.” The unmarked car held four plainclothes police officers, according to the Brooklyn borough president’s office, which has taken an interest in the case.