According to a report in 2001, the collapse of almost all buildings in the WTC compound has been caused by jet fuel fire. Construction steel, though known to be susceptible to heat, is fire-resistant and non-combustible. However, studies have shown that the presence of fire will only significantly reduce the strength of construction steel but not push it to total collapse.
The National Institute of Standards and Technology and other reports have insisted continuously that the World Trade Center Tower 7 is the only building in the world which has experienced total collapse due to fire. Data have shown that a Philadelphian skyscraper burned for 18 hours and a tall building in Spain burned hours on end as well, but both remained standing after the fires.
This structure was situated right across to the Twin Towers and housed offices of important security agencies such as the CIA, Department of Defense, and the like. Analysis of videos taken from the area has shown that the building was on fire from the inside.
With Tower 7 located roughly 100 meters away from the burning twin towers, it raises the question of how the building caught fire from the inside. Other buildings such as the Postal Office beside Tower 7 only suffered damage due to debris and dust but was reopened once again in 2004. Theories have circulated that perhaps Tower 7 was bombed, burning all sensitive data and information from the CIA and Department of Defense.
Perhaps the fate of the Tower 7 can give a clearer explanation to what really happened to the Twin Towers. Evidence to prove such bombings are still being investigated and have not been confirmed so far. But research from the University of Alaska concluded that Tower 7 did not collapse from the fire. A clearer understanding of their statement will be achieved once their study is published.