Three years ago, the National Institute of Standards and Technology began to develop one of the most detailed computer models of a building ever made. It incorporated structural beams and desk chairs, wind speeds and high-level physics. The purpose? To determine the exact cause of the building’s unusual collapse. After all, no building taller than 15 stories had ever succumbed to a structural fire before 9/11. While the authorities and many experts maintained that fire—and no other cause—felled World Trade Center Tower 7, the institute wanted to be certain, in large part to ensure that such a failure would never happen again.

On August 21, the institute released its findings in a detailed, 115-page report, which confirmed that a structural fire was in fact the cause of the collapse. The agency’s investigative team studied other possible scenarios, such as a fuel fire caused by a Con Edison substation, located in the building’s basement; impact damage from Tower 1 debris; and the use of explosive charges. All such means were ruled out.

“We conducted this study without bias, without interference from anyone, and dedicated ourselves to do the very best job possible,” Shyam Sunder, the lead investigator, said at a press conference for the report’s release. “We have had only one single-minded goal during this entire effort. We wanted to determine the probable sequence of events that led to the collapse of Building 7 on 9/11, and then to share that information with the public in order to improve building and fire safety.”

The investigation team, made up of 50 professionals from both the agency and private sector, noted that a critical factor in the building’s fall was the severance of the city’s water main during the collapse of Tower 1. This disabled the sprinkler system, allowing fires on six floors to burn for over seven hours, before Tower 7’s eventual collapse.

However, because no other tall building had ever collapsed due to a fire before, the team still believes it was the building’s design that led to its downfall. “The exterior columns of the building were more closely spaced than the interior ones,” Sunder said. As the fire heated them, thermal expansion caused connections to fail, leading to a progressive collapse that spread from one column to a corner, and eventually to the entire building.

The full report can be found at wtc.nist.gov.