CHICAGO — What if it should happen again?
A decade after the Sept. 11 terror attacks, could any of the nation's 21,000 high-rises withstand an attack like those that caused New York's twin towers to collapse? Could the thousands of people inside find a way to safety?
At Chicago's Willis Tower, like other skyscrapers around the country, much has changed since two hijacked jetliners slammed into the World Trade Center on Sept. 11, 2001. North America's tallest building now has concrete barriers, metal detectors and sophisticated security cameras that trace every movement in and around the structure.
Those measures, however, might do little to prevent a calamity on the scale of Sept. 11. Despite proposals for major structural changes over the past decade, thousands of buildings remain vulnerable, experts say, because the cost to retrofit existing structures is too high, and cities and states have been slow to adopt tougher building codes for new construction.
Other, less sweeping improvements, such as equipping elevators for use in evacuations, are lagging behind other countries, too.
"You only can do as much as lobbyists, politicians, and the agencies you're dealing with will let you do," said Monica Gabrielle, whose husband died in the 9/11 attacks and who co-chairs the Skyscraper Safety Campaign that sprang up afterward. "The further away you get from events, then you become more complacent."
And for all the talk about reinforced security, there is only so much that can be done to protect buildings that stand 1,000 feet (305 metres) or more, which Donald Trump implicitly acknowledged when he decided his new Chicago skyscraper would not climb as high as the Chicago's Willis Tower because he did not want it to become a target.
As for skyscrapers themselves, while there was talk after 9/11 about making them sturdier and easier to escape from in an emergency, the structural work that would have been necessary was either too expensive or just impossible.
"I don't know of any buildings that have gone through a structural retrofit for the purpose of withstanding a major attack like 9/11," said Adrian Smith, an architect who designed the Trump International Hotel and Tower in Chicago and the Burj Khalifa in Dubai, now the world's tallest building.
At the same time, some actions have been taken to make new buildings safer and more secure.
The International Code Commission has recommended 40 post-9/11 building code changes, including wider stairways to ensure firefighters can climb up while occupants are coming down. Municipalities can adopt the changes as they see fit, but they are not mandatory, said ICC spokesman Steve Daggers.
Chicago, for example, adopted an ordinance that requires high-rises to have an emergency evacuation plan on file with the city. And the tallest buildings must provide the fire department with their floor plans so crews know the exact layout of the buildings when they walk in.
People who live and work in high-rises around the city say evacuation drills are now routine, something many say never or rarely happened before 9/11.
Many high-rises also are more difficult to enter. In the immediate aftermath of the attack, Willis Tower installed airport-style security, complete with officers searching bags.
That has been scaled back, said David Milberg, a spokesman for the Schiff Hardin law firm, which has offices a little more than halfway up the 110-story building.
"Now we have keycard access for tenants. Nontenants must produce photo IDs, and we have to register guests in advance," he said.
"It's not as conspicuous as it was, (but) you don't get in here unless you're vetted," he said.
Not surprisingly, new buildings, those under construction and those on the drawing board have a number of features that older buildings did not.
In New York City, for example, stairwell enclosures in high-rises must be wider and made of harder materials, and elevator shafts must be stronger as well.
To prevent the pancaking that happened at the World Trade Center as one floor fell onto another, the city requires high rises to be built to prevent "progressive collapse," but it doesn't spell out how to do that.
Even in places where codes have not been updated, some high-rises are taking steps to strengthen their buildings, said Jon Schmidt, an associate structural engineer and director of anti-terrorism services for the Kansas City, Missouri-based Burns & McDonnell, an engineering, architectural and consulting firm.
Materials and measures once reserved for military and government buildings gradually are becoming more mainstream, including concrete-encased stairwells to protect evacuating tenants and laminated glass that is less likely to shatter into fragments during a blast, Schmidt said.
More attention is being paid to fireproofing material that better sticks to steel — an issue that got a lot of attention because the jets that hit the twin towers apparently knocked the coating off the girders to the point they softened and broke.
But money is never far from mind.
"That's one of the frustrations," says Irwin Cantor, a structural engineer and consultant who has engineered major high-rises nationally and sits on New York City's planning commission.
"A guy says, 'I want to protect my building against a bomb blast of 'X' pounds ... and you tell them, 'I can protect it from a bomb of such-and-such pounds and such and such a distance for $10 (million), $20 million.
"And they say, 'Wait a minute, I'm scared but I'm not that scared," Cantor said.
One major change that experts say is coming is the construction of elevators that can be used in fire emergencies by both people fleeing buildings and firefighters climbing up inside them, a common practice in other parts of the world.
"We like to think of not using elevators in fire emergencies as one of the most successful public education campaigns in history," said Jason Averill, a fire safety expert with the National Institute of Standards and Technology.
It is unclear how many of the 2,700-plus people who perished at the twin towers died before getting to the ground. A study concluded many of them would have survived had they been able to take elevators.
"I'm absolutely convinced it shortens evacuation time to such a degree we have to find ways to embrace the technology," Averill said.
A question looming over all of these preparations and code changes has far less to do with elevators and stairwells than it does with a public's memory of people in the twin towers who perished because voices on the intercom told them to go back to their desks.
There is real concern among safety experts that the next catastrophe will trigger a stampede of workers no matter what they are told, which is one reason why some fire departments, including Chicago's, now take command of a building's intercom system as soon as they arrive.
"There's more trust listening to a representative from the fire department than someone from building security," said Larry Langford, a spokesman for the Chicago Fire Department.
But to some high-rise residents, it won't matter. Nan Feiber, who lives on the 33rd floor of a 57-story Chicago high-rise, said she wouldn't hesitate if the day comes.
"I'm getting the hell out of here."
Associated Press writers Karen Hawkins and Tammy Webber contributed to this report.