Authorities had been trying to evacuate the twin towers of the World Trade Centre in New York when the glass-and-steel skyscrapers came down in a thunderous roar within about 90 minutes of being hit by two airliners in a terrorist attack.
But many people were thought to still be in both buildings, many of them trapped and unable to get out.
The two planes blasted fiery, gaping holes in the upper floors of one of New York's most famous landmarks and rained debris on the streets.
About an hour later, the southern tower collapsed with a roar and a huge cloud of smoke.
The other tower fell about half an hour after that, covering lower Manhattan in heaps of gray rubble and broken glass.
Firefighters trapped in the rubble radioed for help, others stumbled from the scene, coughing and covered in ash.
Dust and dirt flew everywhere and a layer of ash fell for miles around.
There is still on word on the number of fatalities or casualties.