America's sense of security has never been the same since Sept. 11, 2001.
"Every time we fly over ground zero and just the fact that the buildings are not there, it reminds you. You get upset every time you see that giant emptiness," said Fernando Almeida, a detective with the New York City Police Department.
Hovering over ground zero in an unmarked police helicopter -- new since 9/11 -- Almeida showed ABC's Kate Snow features the NYPD relies on to help keep the city safe.
"If we go a little closer, we can go into the infra-red mode and then we can spot any suspicious movement," Almeida said.
The chopper is equipped with a navigation system and a powerful camera.
People on the ground have no way of knowing whether its lens is pointed at them.
Since 9/11, a variety of measures meant to tighten security have been put in place.
The Department of Homeland Security alone spends about $40 billion a year.
Bomb-sniffing dogs patrol sidewalks and airports. Armed guards provide visible displays of force.
In emergency operations centers, federal agents work alongside local cops.
When asked by Snow whether he thought an attack like 9/11 could happen again, NYPD's Deputy Chief Harry Weiden said, "Anything could happen again. But I think we are much more prepared and much more focused right now."
Despite the beefed-up presence of air marshals and reinforced cockpit doors, Americans still know that in the air, there are gaps in security.
"Baggage screening is really not much more effective than it was before 9/11," said James Carafano, a senior research fellow with the Heritage Foundation.
Only 34 airports have high-tech machines to scan for explosive residue.
Terrorists have proven they can attack so-called "softer" targets like trains. Along with bridges, tunnels and ports, they can't always be guarded.
"We inspect very, very, very few of the containers that come into our ports," said Jamie Gorelick, a member of the 9/11 Commission.
"And as you know, our ports are essentially in the middle of our cities," Gorelick said.
Five years after the fall of the World Trade Center, there are still concerns about communication.
"We do not have a radio spectrum that first responders can use to speak to each other," Gorelick said.
Sharing information is essential, and experts say vigilance may be the best defense this country has.
"Sometimes we do get a little complacent," said NYPD Inspector Philip Van Gostein. "It's human nature. But we need to remind people that we're still in the cross hairs of terrorism."