Murder in Blue Light.

Model Starlet, leads a double life; at night, she becomes Sherry, a Times Square prostitute, in order to search for her brother's killer. He was a marine who had been involved in a game of hot potato with a live hand-grenade on a boat returning from Lebanon. The game had ended tragically when the grenade went off, killing three men and injuring six others, castrating one of them. Now someone is stalking New York and stabbing ex-marines to death, leaving fake hand-grenades between their legs after castrating them. Hard-boiled policeman Flanigan thinks Starlet is putting herself in too much danger, so he intentionally exposes her double identity, then poses as her to catch the real killer.