For its new original documentary "Ground Zero Rising: Freedom vs. Fear," CNBC interviewed a wide array of experts involved in the safety and security planning of the World Trade Center – and received exclusive access to the site's operations command center to learn how it keeps tenants and tourists safe.
Rising to a symbolic height of 1,776 feet, One World Trade Center is an emblem of American resilience.
With a cost of $3.9 billion, the tower is also a symbol of Western capitalism. Much of its price tag, however, is attributable to security enhancements that arguably make it one of the safest buildings in the world. Yet, even though One World Trade was specifically designed to withstand a 9/11-type attack or 1993-style bombing, the iconic building likely remains a target.
Shortly after the building opened, Chris Rock joked in his "Saturday Night Live" monologue, "They should change the name from the Freedom Tower to the Never Going in There Tower 'cause I'm never going in there."
But architect David Childs built safety into the core of One World Trade — literally. The tower's signature feature is its unique concrete core, which extends from a 185-foot-tall fortress-like pedestal all the way to the top of the building.
Author and historian Judith Dupre, who followed the rebuilding for years, considers the use of the concrete core to be a technological leap forward. "Because of various union issues in New York City, concrete cores weren't used very much,"she said. "But after 9/11, it was so important to have the most secure building possible that they sat down with the heads of the unions and said, 'Guys, we have to do this differently. We have to do it better.'"
When it comes to One World Trade, better is an understatement — the concrete itself is the most dense and impact-resistant ever used in a building. "The concrete core is 14,000 psi. Strongest concrete ever used anywhere, in a skyscraper," Dupre said.
As the exclusive leasing agent for One World Trade, Tara Stacom's job is to convince prospective tenants that the tower is impenetrable, but escapable.
"All the new buildings today are building based upon what we've done here at One World Trade Center," said Stacom. "So cores are strengthened. They're concrete. Life safety is within those cores. Stairs now are made extra wide. We have something unusual, which is a direct fire-responder stair. So they have their own stair to get up and down in the case of need. They have their own separate elevator as well that can operate."
To prevent a logjam of people trying to get out in case of emergency, One World Trade's staircases are 20 percent wider than required by code.
Safety and security considerations contributed to the high cost not only of One World Trade, but of other nearby buildings. For example, the estimated cost to reconstruct the nearly 200-year-old St. Nicholas Greek Orthodox Church, which was destroyed on 9/11, is $40 million.
"This construction zone is extremely expensive, because of the high security, because of the high tolerances," said Jerry Dimitriou, executive director of administration for the Greek Orthodox Archdiocese of America. "We're sitting on top of a bombproof pad, because of the specifications that the Port Authority has put forth. … The design has to be maintained so that it can withstand certain blast criteria. So, all of those things add up to a more expensive build for the church."
But 15 years after the deadliest terrorist attack in U.S. history, these expensive architectural feats of defiance may be a defense against the wrong threat.
Former FBI Special Agent Don Borelli suggests the greatest danger is posed by potential "lone wolf" attackers. He also believes there have been "lots" of credible threats made on One World Trade.
"Chances are, based on everything that we've seen, it's likely that somebody will take another shot at this building and the surrounding area," he said.
However, Borelli thinks the chances of a 9/11-type attack — a large-scale event such as coordinating the hijacking of multiple planes — are small. "A more likely scenario is somebody that has walked in here with some kind of an automatic weapon and start shooting at people that are just here to enjoy the site in this plaza. Something like a San Bernardino-type of event," he said.
George Anderson, director of security for the World Trade Center, recognizes the need to be constantly aware of the changing threat environment.
"History is but one element of what informs the security plan here at the World Trade Center site," said Anderson, referring to the bombing by a vehicle-born improvised explosive device 23 years ago. "We know that was the history here in 1993, but we have a different threat profile here in 2016. We just need to look at the papers and at the news media to see what's going on around the world."
"We have the lone wolf situation here in this country — you don't need to have been trained directly by ISIS to come here and perpetrate certain types of attacks. So our security program here has to take into account all of those threats, all of those risks, and develop solutions," Anderson said.
On the plaza, a display of force is visible: Officers from the NYPD counterterrorism bureau, New York state troopers and Port Authority police are armed with heavy weapons to serve as a visible deterrent to potential active shooters. The most important security features, however, are hidden.
CNBC gained exclusive access to the WTC Operations Command Center, where Anderson and his team monitor every inch of the 16-acre site. According to Anderson, federal, state, and local homeland security partners were instrumental in developing the security program.
"We are able to use the cameras both reactively to look at incidents that may be happening and also proactively to search for any concerns that we might have," Anderson said.
After the Pulse nightclub massacre in Orlando, Florida in June, an NBC News poll found that half of Americans strongly support extensive surveillance and security checks in public places to prevent terrorist attacks. Another 32 percent somewhat supported such measures to improve domestic security.
The site's vehicle security center, equipped to detect explosives and radioactive material, was designed to prevent another bombing like the 1993 attack under 2 World Trade that killed six people and injured more than 1,000. Drivers are not allowed to park next to the towers or museum, and all deliveries are made using roads hidden underground.
"We screen every single vehicle that comes onto the site from delivery vehicles to tour buses to privately operated vehicles," said Anderson. "We have no public parking that's available here at the World Trade Center site — all the vehicles that come here are known to us [and] we know who the drivers are going to be."
"It truly is unique because I'm not familiar with any other screening center for a commercial real estate complex that is like the vehicle screening center here at the World Trade Center complex."
Even with extensive security measures in place, Borelli emphasized the need to stop perpetrators in the planning stage. "Security is not just gates and guards and cameras and all that," he said. "Security's intelligence. Security is knowing who might be driving down that street long before they get there. Who are the people that are looking at this as a potential target? And trying to dismantle those groups or disrupt those potential plots."
For David Dunlap, a veteran New York Times reporter who has covered the rebuilding for over a decade, the WTC's show of force is also a show of vulnerability. "So much of the security apparatus we develop, it seems … is meant to project our strength. But what it says is, 'Yikes,'" said Dunlap. "Ultimately, both the Trade Center and the Stock Exchange will never be fully integrated because the physical barriers always say, 'This is a danger zone.' We're scared something really bad could happen here."
Borelli, however, thinks the WTC site strikes a good balance between fortification and freedom.
"Is it 100 percent safe? No. But is it a good trade-off between being safe, and also aesthetically, and letting people be able to move around and enjoy it? I think so," he said. "You look at all of the people that want to visit this beautiful site, and visit New York City and Times Square and everything else. And how do you protect all of those people without just being in some kind of a lockdown state? You really can't."
"We could be [targets]. But we don't want to live like that. I mean, I think there's this trade-off between security and our freedom of movement."