In November of last year, rookie NYPD cop Peter Liang shot and killed Akai Gurley in the stairwell of the Brooklyn public housing building where he lived. Four months later, a retired correction officer named Willie Groomes shot and killed Gilbert Drogheo on a Brooklyn subway platform. Because Groomes wasn’t indicted for his killing, Liang is arguing in court, he shouldn’t be indicted either.

On Tuesday evening, 69-year-old William Groomes, a retired corrections officer, was riding a Brooklyn-bound 4 train when he got into a confrontation with two younger men. The specifics of their quarrel aren't totally clear, but two things are certain: Groomes shot Gilbert Drogheo dead, and he wasn't arrested for it. Why not?