REAGAN WASHINGTON NATIONAL AIRPORT, Va. (WUSA9) -- The shooting and bombing in Istanbul has the Department of Homeland Security re-evaluating what it's doing to keep US airports safe, but sources said there is no sign of a specific threat here.

Homeland Security spokeswoman Marsha Catron said the department is adapting "both seen and unseen security measures in order to counter evolving threats."

Heavily-armed Airports Police were patrolling the terminal here to deter terrorists -- and ease the fears of travelers.

"You feel a little safer," said traveler Noellia Gonzalez, who was startled when three officers with rifles walked by her. "I'm nervous everywhere," said Shirley Muraca, who was flying in to see her daughter..

In Istanbul, the killers were able to make it into the arrivals hall and into a parking lot. There is some screening outside each of those areas. And the terrorists failed to make it through the final checkpoint.

There's been a lot of talk about pushing security perimeters farther away from the airport. But the hard truth is, wherever you put a checkpoint, people and traffic are likely to back up, creating a potentially attractive target for terrorists.

"It can be perhaps seconds after somebody gets out a car, walks into the public side of the terminal, and starts the attack," said former Transportation Security Administrator John Pistole. "It's very difficult to stop in each and every instance."

Across the country, police are doing their best. In New York, more tactical weapons. In Miami, more curbside patrols. In LA, security had already been stepped up ahead of the busy Fourth of July weekend. And at Dulles, they're promising a robust security structure.

"It's just scary," said Rita Barnum, who was flying out for a vacation in California. "You just kind of want to watch what's going on around you. It's a scary time to fly."

At Reagan Washington National, engineer James Ball was unsure if it was all working.

"I had a buck knife I forgot in a case, and it went through screening and I didn't find it until the other end. And I thought, "How the heck could something like this get through?'"

But also felt in some way on this Fourth of July weekend, it was important to put our fears aside.

"We have to go on living," he said. "Otherwise they win."