New York City officials have beefed up security on the city's subways because of what they call a specific threat to the subway system.
At a news conference Thursday, New York Mayor Michael Bloomberg said the threat originated overseas and was the most specific threat the city had received to date.
An FBI official Mark Mershon said the threat may have been "partially disrupted" by classified operations. But New York Police Chief Ray Kelly said officials were sufficiently concerned to further increase the police presence in the subways.
He said police will be paying special attention to briefcases and baby carriages.
Mayor Bloomberg said New Yorkers should go about their normal business but report any suspicious activity.
New York has one of the world's largest subway systems, with more than a thousand kilometers of track and more than four-and-a-half million riders each day.
Bombings on the London subway system July 7th killed 56 people, including four suicide bombers.