A third plane struck the Pentagon in Washington, causing the evacuation of the military and intelligence building as it burned. Here are the tragic moments in pictures:

Courtesy of BBC News

Smoke billowed out of the World Trade Center after the first plane hit it	Less than 18 minutes later, another plane closed in on the other tower
The plane seemed to swerve to hit the other tower	The second plane caused a massive explosion and fire on the second tower
The South Tower collapses in a cloud of debris and smoke	The collapse obscures much of downtown Manhattan
Moments later much of the landmark disappeared behind smoke	The collapse of the second tower, the West Tower, followed shortly afterwards
Shocked people at St Patrick's Cathedral watch the World Trade Center collapsing	The Pentagon was also hit by another aircraft minutes after the World Trade Center explosions
Part of the Pentagon collapsed after the plane struck	Firefighters battled to control the blaze after efforts had been made to evacuate the building
George Bush was told the news as he spoke to children in a Florida school	Smoke was visible across the whole of Manhattan island