Biden and Obama to attend memorials marking 20th anniversary of 9/11

• President to be joined by predecessor in downtown New York
• Memorials also planned in Shanksville and at Pentagon

The sombre ceremonies that take place in New York, Pennsylvania and on the outskirts of Washington every September 11 to mark the 2001 terrorist attacks will take on an extra significance on Saturday morning, as America commemorates the 20th anniversary of the al-Qaida hijackings.

Many family members of the nearly 3,000 victims killed on that day are expected to gather in downtown Manhattan, in a field in Shanksville, and at the Pentagon, where the terrorists crashed the passenger jets they had seized that morning.