It continues to be an opportunity for lessons to be learned.
In a batch of government reports released Tuesday, engineers say they know why the twin towers fell down, how things went wrong during the evacuation, and believe new thinking is needed on how to evacuate people from endangered skyscrapers and how to get rescuers into them more quickly.
The reports by the National Institute of Standards and Technology also detail how early decisions played a key role in determining who died and who survived.
A total of 2,749 lives were lost when the two hijacked jets were crashed into the twin towers.
The NIST reports note that expectations of how quickly people move down stairwells have been based on "phased" evacuations, and not the full-scale evacuation of the type attempted at the World Trade Center.
"The average surviving occupants moved slower down stairs and through stairwell exits than previously recorded for a non-emergency evacuation," investigators concluded.
To underline the importance of the finding, NIST estimated that had the buildings been hit at a time when they were full, as many as 14,000 people may have died.
In Tower 1, the average survivor took 48 seconds to descend a flight of stairs, or about half the slowest evacuation speed calculated in a current fire safety handbook used by engineers in designing buildings, the report found.
The briefing in New York was conducted by Shyam Sunder, the lead investigator for the agency's building and fire safety investigation into the disaster.
The buildings would not have collapsed if the fireproofing material surrounding the steel had not been stripped away by the impact of the planes, Sunder said.
Without that stripping effect, the intense heat of up to 800 degrees Fahrenheit would not have been enough to bring the buildings down. Sunder added, however, that it would not have been reasonable for engineers to have installed fireproofing designed to sustain the impact of a fuel-laden jetliner.
The thickness of the fireproofing had been called into question during the course of the two-year investigation; Sunder said far more important was the material's ability to stick to steel. Now there are other ways to put on fireproofing to make it adhere to the steel better.
David Collins, a member of the advisory committee that offered suggestions and questions to NIST investigators, said the research showed design and construction of the building were not major contributors to the collapse.
"I think everyone took deliberate steps to try to do what was necessary to make the buildings as safe as possible," said Collins, a Cincinnati-based architect.
NIST also found the much-documented problems with radio communication and information-sharing among first responders probably led to deaths among emergency personnel.
"Lack of timely information sharing and inadequate communication capabilities likely contributed to the loss of emergency responder lives," according to the draft report.
Sunder said some of the firefighters they interviewed said they felt they would have gotten better real-time information at home watching TV than what they were told at the scene.
The findings are NIST's last step before issuing its final recommendations in June, the culmination of exhaustive research and testing that produced 10,000 pages of data.
The other findings - about the emergency response and the behavior of those who were in the building - will be part of the ongoing debate over how to improve skyscraper safety.
Investigators have determined that previous expectations about how long people would take to evacuate buildings were not borne out by events at the World Trade Center.
The report also noted that some people delayed their evacuation by "milling" in offices, deliberating about what to do, or debating how to find the next stairwell.
The previous evacuation models cited in the report are important because architects use them to calculate the capacity needed in stairwells, elevators, and other means of exiting a building.
The report also emphasizes the limited ability of rescue personnel to reach higher floors quickly to battle fires and rescue trapped civilians.
That proved critical for firefighters who climbed 70 flights of stairs carrying up to 100 pounds of gear - and then tried to battle flames or clear debris once there.
Those concerns are spurring a debate both within the NIST group and among the larger fire rescue and construction fields about stairwell and elevator design.
The debate centers around whether "fireproof" elevators, designed to resist flames and smoke, should be installed in new buildings, particularly those that rise above 40 or 50 stories, and the best width and location of stairwells.
Elevators played a critical, but contradictory, role. In some cases, they helped significant numbers of people get out quickly. For others, they became sealed containers trapping them inside a doomed building.
NIST's ultimate goal is to recommend building code improvements.
By Devlin Barrett and Karen Matthews