New York City officials have stepped up security in the city's massive subway system after receiving what the mayor called a "specific threat" of a terrorist attack.
Mayor Michael Bloomberg said late Thursday at a hastily arranged news conference that officials had "never before had such a specific threat" to the subway system.
An FBI official, Mark Mershon told reporters that the threat was "partially disrupted," and that information about the threat was linked to the arrest of three people in Iraq.
Mayor Bloomberg said officials have been aware of the threat for a few days, but did not make an announcement in order not to jeopardize the lives of agents investigating the plot.
In Washington, a Homeland Security official, Russ Knocke downplayed the seriousness of the threat, saying the information received "is of doubtful credibility."
Police have increased patrols in the country's largest city after receiving information that as many as 20 people might be involved in a plot to hide explosives in baby strollers, briefcases and packages on the New York subway system.