By Jeremy Cooke
BBC News, New York
A federal inquiry into the 11 September collapse of the World Trade Center calls for major changes in the safety specifications of US skyscrapers.
The report follows a three-year study into the towers' collapse
One of the key recommendations of its report is for all new tall buildings to have better emergency exits.
Just under 2,750 people died when the Twin Towers came down.
Some 12,000 more would have died had the towers been full, partly because of poor emergency exits, says the National Institute of Standards and Technology.
Only about a third of the World Trade Center employees had arrived at their desks when the buildings collapsed.
One of the key findings of this three-year investigation is that wider exits must be provided, with entrance doors on every floor.
Other recommendations include the need to construct skyscrapers to withstand what is called progressive collapse - in other words to avoid the sort of cascade effect which ultimately brought down the Twin Towers.
These new federal recommendations will not be binding and they will certainly be resisted by many in the construction industry.
The improved specifications would add an estimated 5% to the cost of building a new skyscraper.
Some engineers are warning that it is practically impossible to build a structure capable of withstanding the sort of catastrophe which befell the World Trade Center on 11 September 2001.