In the wake of a continuing terrorism probe, the Metropolitan Transportation Authority has stepped up patrols at "key locations" in the city, the agency announced Tuesday.

MTA officials said while there has been no conceivable threat to the city's subway system and commuter trains, extra officers will be on hand - at places like Penn Station and Grand Central Terminal - incase an unfavorable incident occurs.

Nationwide, the FBI and Homeland Security have issued security bulletins regarding terrorist interest in attacking sports stadiums, entertainment venues and hotels. In a bulletin issued Friday, officials warned that more security should be put into place at mass transit systems across the United State.

They said homemade explosive devices have been the most common ways to blow up railroads and other mass transit system overseas.

Police Commissioner Ray Kelly would not comment on the probe Tuesday but said transit officials are in close communication with the FBI- NYPD task force.

Three people - 24-year-old Najibullah Zazi, his father Mohammed Wali Zazi, and Ahmad Wais Afzali, a Queens imam - have been charged with lying to officials investigating the alleged New York terror plot.