Threatening messages have been posted on the Facebook page of the White House. "We'll come back U.S.A. One day only 11/9/2011," says one message, featuring a photo of Osama bin Laden. "We'll come to u white house sooooooooooon," says another. "We'll come back 11/9/2011 to kill u all," a third posting reads. Meanwhile, a White House counterterrorism adviser says the Obama administration is taking "very seriously" the threat of a possible Qaeda plot on Sunday's 9/11 anniversary. John Brennan told Fox News Sunday that investigators are trying to piece together the details of an intelligence tip on a possible car-bomb attack that was announced Wednesday. But officials have said they've found no evidence that any of the three suspects has sneaked into the country.