Thursday marks the seventh anniversary of the September 11th, 2001 terrorist attacks on New York City and the Pentagon in Washington D.C.

On that morning, terrorists hijacked four commercial passenger jet airliners. The hijackers intentionally crashed two of the airliners into the Twin Towers of the World Trade Center in New York City, killing everyone on board and many others working in the building, causing both buildings to collapse within two hours, destroying nearby buildings and damaging others.

The hijackers crashed a third airliner into the Pentagon. The fourth plane crashed into a field near Shanksville in rural Somerset County, Pennsylvania, after some of its passengers and flight crew attempted to retake control of the plane, which the hijackers had redirected toward Washington, D.C. There are no known survivors from any of the flights.

Excluding the 19 hijackers, 2,974 people died in the attacks. Another 24 are missing and presumed dead. The overwhelming majority of casualties were civilians, including nationals of over 90 different countries.

New York is preparing to mark the sevenths anniversary of September 11th Thursday with new appeals for heightened vigilance for new terrorist attacks. The Presidential candidates of the two major parties - the Republican John McCain and the Democrat Barack Obama are scheduled to participate at the traditional ceremony held each year at "ground zero" - the location of the collapsed Twin Towers. As each year, the names of all those who died in the attacks are going to be read during the ceremony.

Both Presidential candidates have agreed to put their election campaign TV ads on hold Thursday as a sign of respect for the victims of the terrorist attacks.

The focus of this year's commemoration is teaching the new generations about the lessons of September 11th and portraying how the families of the victims cope with the loss of their loved ones seven years later.