Fanning out along the platform at the City Hall subway station in Manhattan, the plainclothes police officers blended in with the morning commuters. As they prepared to board a northbound train, they watched closely, their eyes darting from rider to rider.

The team of seven officers was searching for men who use the subway’s crowded confines to get too close to women. Some furtively touch female passengers, while others rub up against the women they have targeted.

At Union Square, Detective Marquis Cross saw a man he recognized from a previous sex-offense arrest standing suspiciously close to a female passenger. As the woman left the train, Detective Cross jogged after her to ask if she had felt anything unusual. She said that she had not, but that the man did seem too close.

The officers, all from the New York Police Department’s Transit Bureau, were working the Lexington Avenue line, one of the most overcrowded in an increasingly crowded system. The ever-tightening crush of passengers provides easy cover for men who prey on women, the police say.