One World Trade Center, rising to a symbolic height of 1,776 feet, cost of $3.9 billion to build.  Much of its price tag 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.

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.