Threats posted on White House Facebook page

"We'll come back U.S.A. One day only 11/9/2011," says one message, featuring a photo of Osama bin Laden, using the date/month formula to reference Sept. 11.

"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. The posts appeared briefly and were taken down.

Secret Service spokesman Ed Donovan said the agency has referred the messages to its internet threat desk.

"They process it, make an assessment and determine what the next step is," Donovan said.

New York City and Washington, D.C., have tightened security after intelligence collected from overseas indicated a possible threat involving car bombs, as well threats to bridges and tunnels. The information indicated that three men would travel from Pakistan to the U.S. to carry out an attack.