US On Alert

New York City Police officer King Tung, right, stands outside the rotunda entrance to Citi Field as fans enter the stadium before a baseball game between the New York Mets and the Chicago Cubs, Friday, Sept. 9, 2011, in New York. The latest threat, potentially targeting New York or Washington, prompted an even greater security surge in those cities. Law enforcement around the country had already increased security measures at airports, nuclear plants, train stations and more in the weeks leading up to the 10th anniversary of the Sept. 11 attacks. (AP Photo/Kathy Kmonicek)

A New York City Police officer stands outside Citi Field as fans enter the stadium before a baseball game between the New York Mets and the Chicago Cubs, Friday, Sept. 9, 2011, at Citi Field in New York. Security has been enhanced around the country in the weeks leading up to the 10th anniversary of the Sept. 11 attacks. (AP Photo/Kathy Kmonicek)

New York City Police officer Steve Chei, right, patrols outside Citi Field as fans enter the stadium before a baseball game between the New York Mets and the Chicago Cubs, Friday, Sept. 9, 2011, in New York. Security has been enhanced around the country in the weeks leading up to the 10th anniversary of the Sept. 11 attacks. (AP Photo/Kathy Kmonicek)

A New York police officer examines the rear section of a truck at a vehicle check point on Friday, Sept. 9, 2011 in New York. The city is deploying additional resources and taking other security steps in response to a potential terror threat before the 10th anniversary of the Sept. 11 attacks. U.S. counterterrorism officials are chasing a credible but unconfirmed al Qaida threat to use a car bomb on bridges or tunnels in New York or Washington. (AP Photo/Jin Le

In this photo provided by the New York City Mayor's office, Mayor Michael Bloomberg, left, rides the subway to work Friday, Sept. 9, 2011. Speaking Friday morning in the Brooklyn Bridge City Hall subway station, Bloomberg said he had no new information about a terror threat to New York since his Thursday night public briefing with the head of the FBI's New York division. He said he hopes New Yorkers will go about their everyday business. (AP Photo/NYC Mayor's Office, Spencer T. Tucker)

A New York City police officer examines a commercial vehicle at a checkpoint on 59th St. and Park Ave. on Friday, Sept. 9, 2011 in New York. The city is deploying additional resources and taking other security steps in response to a potential terror threat before the 10th anniversary of the Sept. 11 attacks. U.S. counterterrorism officials are chasing a credible but unconfirmed al Qaida threat to use a car bomb on bridges or tunnels in New York or Washington. (AP Photo/Jin Lee)

A piece of twisted steel salvaged from ground zero sits nearby as Secretary of State Hillary Clinton gives a speech on counterterrorism at John Jay College of Criminal Justice, Friday, Sept. 9, 2011 in New York. Her visit comes amid heightened security due to a credible but unconfirmed terror threat against New York and Washington, just before the 10th anniversary of the terrorist attacks. (AP Photo/Jason DeCrow)

A heavily armed Port Authority police officer stand guard next to the North Pool at the World Trade Center memorial site Friday, Sept. 9, 2011 in New York. Just days before the 10th anniversary of the Sept. 11 attacks, U.S. counterterrorism officials are chasing a credible but unconfirmed al Qaida threat to use a car bomb on bridges or tunnels in New York City or Washington. (AP Photo/Mary Altaffer)

A New York police officer examines a SUV at a vehicle check point on Friday, Sept. 9, 2011 in New York. The city is deploying additional resources and taking other security steps in response to a potential terror threat before the 10th anniversary of the Sept. 11 attacks. U.S. counterterrorism officials are chasing a credible but unconfirmed al Qaida threat to use a car bomb on bridges or tunnels in New York or Washington. (AP Photo/Jin Lee)

An Amtrack police officer stands guard at a track entrance at Pennsylvania Station on Friday, Sept. 9, 2011 in New York. The city is deploying additional resources and taking other security steps in response to a potential terror threat before the 10th anniversary of the Sept. 11 attacks. U.S. counterterrorism officials are chasing a credible but unconfirmed al Qaida threat to use a car bomb on bridges or tunnels in New York or Washington. (AP Photo/Jin Lee)

An Amtrack K 9 police officer checks commuter bags as commuters get ready to board their train at Pennsylvania Station on Friday, Sept. 9, 2011 in New York. The city is deploying additional resources and taking other security steps in response to a potential terror threat before the 10th anniversary of the Sept. 11 attacks. U.S. counterterrorism officials are chasing a credible but unconfirmed al Qaida threat to use a car bomb on bridges or tunnels in New York or Washington. (AP Photo/Jin Lee)

A Transportation Security Administration agent examines a passenger's bag before he boards a train at Pennsylvania Station on Friday, Sept. 9, 2011 in New York. The city is deploying additional resources and taking other security steps in response to a potential terror threat before the 10th anniversary of the Sept. 11 attacks. U.S. counterterrorism officials are chasing a credible but unconfirmed al Qaida threat to use a car bomb on bridges or tunnels in New York or Washington. (AP Photo/Jin Lee)

A police officer with a bomb sniffing dog patrols the Times Square subway station, Friday, Sept. 9, 2011, in New York. Just days before the 10th anniversary of the Sept. 11 attacks, U.S. counterterrorism officials are chasing a credible but unconfirmed al Qaida threat to use a car bomb on bridges or tunnels in New York City or Washington. (AP Photo/Mary Altaffer)