Amid a heightened state of security in New York and Washington during 9/11 memorial events shaded by a new credible terror threat, threatening messages have been posted on the White House Facebook page.

We'll come back to the U.S.A. One day only 11/9/2011, said one message, accompanied with a photo of Osama bin Laden.

Another message read: We'll come to u white house sooooooooooon.

A third message read: We'll come back 11/9/2011 to kill u all.

New York and Washington remain in a high security mode Saturday as authorities try to sort through details of a credible 9/11 terror threat as 9/11 a memorial continues in New York amid high security, but with no reported terrorist activity in the cities.

However, security has been greatly increased on the memorial day in New York and Washington, D.C. -- both sites of the Sept. 11, 2001 deadly terrorist attacks by al-Qaida -- amid news that an informant considered reliable told U.S. intelligence officials of a plot to detonate a car bomb in the U.S. on the 10th anniversary of 9/11 attacks.

in New York Sunday morning, bells chimed at 8:46 a.m. during the memorial, signifying the moment 10 years ago when the first jetliner struck the World Trade Center's North Tower. The crowd surrounded in tight security stood quietely in a moment of silence amid the chimes, which ran again at 9:03 a.m., marking the impact of the second plane which fly into the World Trade Center's South Tower.

Later in the ceremony, Paul Simon played a rendition of The Sound of Silence.

Former President George W. Bush, in office at the time of the 9/11 attacks, read the words of another former president -- Abraham Lincoln -- whom Bush said understood the cost of sacrifice, and reached out to console those in sorrow as best he could. 

In Washington on Sunday, 1,600 people including 100 survivors of the 9/11 attack gathered at the Pentagon. A large American flag hung on the spot where the plane struck the Pentagon, as wreaths were placed on each individual bench at the 9/11 memorial in honor of those who died that day.

New York and Washington were put on heightened security alert late this week as authorities received an intelligence report from credible but unconfirmed sources said that al-Qaida was planning an attack on the 10th anniversary of 9/11. The terrorist organization carried out the deadly 9/11 attacks on New York, Washington and the U.S., killing thousands while destroying the World Trade Center in downtown New York.

We have already had a full complement of people working shifts because of the Sept. 11 anniversary prior to this, Jim Margolin, a spokesman for the Federal Bureau of Investigation's New York office, told Bloomberg. We are taking the logical investigative measures to assess this threat.

Officials have been searching for evidence or individuals related to a tip that al-Qaida might have sent three men to the U.S. to detonate a car bomb on or around the 9/11 anniversary.

On Friday, Vice President Joe Biden said Friday the first active plot timed to coincide with the commemoration of the 10th anniversary of al-Qaida's deadly attack on America is a real threat.

Appearing on CBS' The Early Show, Biden said intelligence investigators are following every possible lead in the attempt to diffuse the threat. Biden said, however, that Americans should be on alert as the 10th anniversary commemorative event nears on 9/11.

People should be alert, they should not alter what they're doing, he told CBS, noting we have significant security, local police and federal agencies working on this. 

Biden said in another interview on Good Morning America that a car bomb might be involved in the threat.

We do have talk about using a car bomb. We've been told that was an intention... from a credible source, Biden said on Good Morning America. But we do not have confirmation of that. 

New York Mayor Michael Bloomberg rode the subway Friday morning in a rare public transportation trip in the effort to reassure the city's eight million residents.

We don't want al-Qaida or any other organization ... to take away the freedoms without firing a shot, Bloomberg told The Associated Press, getting off at the City Hall stop in lower Manhattan near the Brooklyn Bridge. Bloomberg urged New Yorkers to just go back to work. And leave it to the professionals.

U.S. officials said Thursday they were chasing down a credible but unconfirmed al-Qaida threat to use a car bomb on bridges or perhaps tunnels in New York or Washington.

We have received credible information very recently about a possible plot directed at the homeland that seems to be focused on New York and Washington, a senior administration official told CNN on Thursday. 

New York police have said in light of the credible threat that they are beefing up security at bridges and tunnels, and setting up vehicle checkpoints. Police are also implementing bomb sweeps of parking garages and towing more illegally-parked cars.

New York commuters have been told they will see a show of force at major transportation terminals, including Grand Central, Penn Station, and near the Port Authority and Times Square subway stations.

Officials suspect that al-Qaida's new leader, Ayman al-Zawahri, was involved in planning the latest attack threat.