Not easy. Unless/until the act has occurred.
Whatever the warning says, I'd be very surprised if - knowing al Qaeda - they are not going after the President and his family. They'll want their revenge for Bin Laden.
As this lot of thugs are not nearly as bright as but are just as nasty as the folks a decade ago, I'd expect the attempt to be a bomb of some kind, presumably suicide-related. Or maybe a short range missile on Air Force One. Something they know how to do as a result of Afghanistan and Iraq.
Other potential targets - gotta be trains, after Madrid and London. And also churches - as the 11th is a Sunday.
If the secret services have no idea, I'd try to deconstruct the plot based on the way these people think and what they'd want to do. And then I'd get the eyeballs of America engaged in the hunt.
Hope they catch these guys.
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