Scientists and engineers, such as Clemson University Professor of Engineering Dr. Judy Woods and BYU Professor of Physics Dr. Steven Jones, have raised compelling questions about the official account of the collapse of the three WTC buildings. The basic problem for the government's account is that the buildings are known to have fallen at free-fall speed, a fact that is inconsistent with the government's "pancaking" theory in which debris from above collapsed the floors below. If the buildings actually "pancaked," then each floor below would have offered resistance to the floors above, and the elapsed time would have been much longer.
These experts have also calculated that the buildings did not have sufficient gravitational energy to accommodate the government's theory of the collapse. It is certainly a known and non-controversial fact among physicists and engineers that the only way buildings can collapse at free fall speed into their own footprints is by engineered demolition. Explosives are used to remove the support of floors below before the debris from above arrives. Otherwise, resistance is encountered and the time required for fall increases.
The basic problem for the the conspiracy clowns is that the buildings didn't fall at free-fall speed. Just look at the first photo, the debris from the building is falling faster than the building itself -- that's why you see parts of the building from above overlapping the lower floors.