Authorities are looking for two or three possible suspects with a credible, but unconfirmed, terror threat targeting New York City or Washington, DC.

FOX News Radio's Jane Metzler has the details in our ongoing series on national security:

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STATEMENT BY SECRETARY OF HOMELAND SECURITY JANET NAPOLITANO URGING PUBLIC VIGILANCE

"As we head into the 9/11 anniversary weekend, we continue to urge the American public to be vigilant and report any suspicious activity to law enforcement authorities. Simply put, if you see something, say something. We take all threat reporting, including the recent specific, credible but unconfirmed threat information, seriously. We continue to be in close contact with our federal, state and local law enforcement partners to ensure that all steps necessary to mitigate any threats are taken. Our security posture includes a number of measures both seen and unseen and we will continue to respond appropriately to protect the American people from an evolving threat picture both in the coming days and beyond. Homeland security is a shared responsibility, and everyone plays an important role in helping to keep our communities safe and secure."