Threatening messages on the White House Facebook page have promoted authorities to heighten alerts for possible car bombs in Washington D.C. and New York.
The National Terror Alert Response Center said the threats were “specific” and "credible” although not confirmed.
The Federal Bureau of Investigations (FBI) said it has not found any unusual purchases of chemicals or other materials that could be used for manufacturing bombs, but a security bulletin last week warned that Al Qaeda may be planning to attack with bombs in vehicles.
Three terrorists from Pakistan may be traveling to the United States or may have arrived to carry out an attack, officials said.
Last year, a terrorist's gas canisters were found in Times Square in New York.
Messages on the White House Facebook page have been taken seriously. One warned, "We'll come back U.S.A. One day only 11/9/2011," the European style of denoting the date September 11.
"We'll come to u white house sooooooooooon," said another post, and a third one stated, "We'll come back 11/9/2011 to kill u all.”
All of the posts were removed soon after they appeared and are being investigated by the Secret Service.
"They process it, make an assessment and determine what the next step is," Secret Service spokesman Ed Donovan told NBC.
Surveillance also is in force at bridges, tunnels and subways, and a national alert was in effect on Sunday, the 10th anniversary of the Al Qaeda attacks on the World Trade Center and the Pentagon. A fourth aerial suicide team failed to reach its target, the White House, on that day.
Federal agents have questioned more than 100 people after scouring airline passenger records, but they did not discover any direct links with intended terrorist attacks or plots.