NY Subway Identified As Possible Terror Target -- System On Alert

"Law enforcement sources told CNN the threat information came from Iraq and that it included claims that a group of 15 to 20 people were in the United States to carry out an attack against New York's mass transit system.

We have never had before a specific threat to our subway system," Bloomberg said, adding that he still felt secure enough to take the subway home Thursday night.

An estimated 4.5 million passengers ride the New York subway on an average weekday. The system has more than 468 subway stations. In July, the city began random subway searches following the London train bombings."