There was an investigation. But the basic physics chain of events was discovered to be:

Debris from the collapse of WTC 1, which was 370 feet to the south, ignited fires on at least 10 floors in the building at its south and west faces. However, only the fires on some of the lower floors-7 through 9 and 11 through 13-burned out of control. These lower-floor fires-which spread and grew because the water supply to the automatic sprinkler system for these floors had failed-were similar to building fires experienced in other tall buildings. The primary and backup water supply to the sprinkler systems for the lower floors relied on the city's water supply, whose lines were damaged by the collapse of WTC 1 and WTC 2. These uncontrolled lower-floor fires eventually spread to the northeast part of WTC 7, where the building's collapse began.