Timed with Ramadan and the Jewish High Holy Days. "Sources: Subway threat made," from [1] Newsday:

A "credible threat" to the subway system has prompted a vast mobilization of police officers, law enforcement sources said today.

Hundreds of officers were expected to be dispatched as early as this afternoon to every station in Manhattan -- and possibly system wide -- to thwart the attack, which was said to timed to the Jewish High Holidays and the start of the Muslim holy month of Ramadan.

Scores of officers from various narcotics units are being called in to assist in patrolling the subways, beginning as early as this afternoon. Many were expected to convene first at the Brooklyn Navy Yard.