The 47-storey third tower, known as Tower Seven, collapsed seven hours after the twin towers.
Investigators are expected to say ordinary fires on several different floors caused the collapse.
Conspiracy theorists have argued that the third tower was brought down in a controlled demolition.
Unlike the twin towers, Tower Seven was not hit by a plane.
The National Institute of Standards and Technology, based near Washington DC, is expected to conclude in its long-awaited report this month that ordinary fires caused the building to collapse.
That would make it the first and only steel skyscraper in the world to collapse because of fire.