Unimaginable on Sept. 11, investigators now know much more about the anatomy of the World Trade Center towers collapse.

"The towers survived the impacts. It was the fires in addition to the very severe impact that finally brought the towers down," said Gene Corley of the American Society of Civil Engineers.

Corley and others from ASCE worked with the Federal Emergency Management Agency to answer congressional questions about how the skyscrapers were brought down by the violent assault by two Boeing 767s on Sept. 11.

The north tower, struck first, remained standing for 102 minutes. The south tower fell 56 minutes after impact. More than 25,000 people escaped from the 110-story buildings, but 2,823 perished, including 343 emergency responders.

The report concluded that the buildings exceeded safety requirements and could have survived the crashes. They couldn't however, survive the spreading jet fuel — approximately 10,000 gallons in each plane — and immense heat caused by the fire that melted the steel framework.

"The fact that the structures were able to sustain this level of damage and remain standing for an extended period of time is remarkable and is the reason that most building occupants were able to evacuate safely," according to the report released Wednesday by the House Science Committee.

The report describes exactly how each plane impacted the towers, how floor trusses near the impact collapsed, and how crucial fireproofing material was blown off of support beams.

"Up until 9/11, we had never had a collapse of a protected steel building," said Dr. Arden Bement of the National Institute of Standards and Technology.

The report is full of photographs and diagrams of the World Trade Center, and makes specific recommendations for protecting tall buildings. Among them: enhanced fireproofing and larger stairwells so more people can make it out in the event of a catastrophe.

But for many people, in particular families of World Trade Center victims, this report falls very short.

"For no good reason, nearly 3,000 people were killed, not by a fire — killed by a building," said Sally Reganhard, whose son was a rescue worker killed in the building collapse.

"Some of the most vexing questions may never be answered. Thousands of tons of steel were carted away and recycled before any expert could examine what could have been telltale clues," said Rep. Anthony Weiner, D-N.Y., whose Brooklyn district was home to many victims and their families.

The National Institutes of Standards and Technology is set to take over the WTC collapse investigation, which will likely take two years and cost $16 million. NIST could eventually make broad recommendations to the building and fire codes using the trade center findings.

Congress will soon consider having just one agency coordinate investigations into building catastrophes, much the way the National Transportation Safety Board investigates plane and train crashes. The agency will assess threats to structures and emergency evacuation plans for buildings at risk of attack.

What the collapse investigation may never find out is whether the steel supports — trusses — will ever be safe from fiery infernos fueled by aircraft fuel, especially since the fireproofing material required to withstand the impact of an airplane could make construction cost-prohibitive.

Families, however, remain hopeful that the investigation will yield information to prevent future disasters.

"If we have a proper investigation right now with the proper funding, with the proper manpower, with the proper support services, we will find out, we have to find out," Reganhard said.

The Associated Press contributed to this report.