JACKSONVILLE, Fla. - Tuesday's airport attack in Brussels highlights one of the most vulnerable stages of aviation security: the time travelers spend between the curb and the checkpoint.
As travelers wait first to check luggage and then go through metal detectors, they crowd together in areas that are usually lightly patrolled and accessible to nearly anyone.
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While the Department of Homeland Security said there is no specific, credible threat to the United States, airports and subway stations in many large American cities was increased Tuesday.
Officials confirm that is also true at Jacksonville International Airport.
Debbie Jones/community relations administrator for JAX aviation Authority
"Passengers will see an increase(d) presence of police officers," said Debbie Jones, with the Jacksonville Aviation Authority. "We maintain diligence in and around the airport, and we ... encourage everyone who comes to the airport to do the same."
For more than 40 years, security officials and terrorists have been fighting to stay ahead of each other. When airlines and governments made it harder to hijack planes, terrorists found new ways to destroy aircraft. They put bombs in checked luggage until bag screening became standard. The 9/11 hijackers defeated 2001 passenger-screening measures and used knives to turn jets into weapons.
Security checkpoints are designed to keep terrorists and weapons off planes, and for the most part they have worked since the September 2001 attacks.
But along the way, the airport itself became a target.
Rick Parker, former homeland security chief for the Jacksonville Sheriff's Office, viewed video of the Brussels airport attack and said was surprised by the lack of immediate intervention.
"I think the instinct is to react to the chaos around you," Parker said. "If you're not prepared, you're going to take longer to evaluate. If you're prepared you're already going to take action."
Parker said there are eyes and ears everywhere at Jacksonville International Airport.
"There are layers of security," Parker said. "It's law-enforcement sensitive. They don't want it openly discussed, but there are cameras galore and rapid response teams there, and they don't want to be seen."
On Tuesday, terrorists set off two bombs in the departure area of the Brussels airport and another in the subway, killing at least 31 people and wounding dozens. Other bombs were found and deactivated, which is why Parker said people should run, just in case another threat is nearby.
"We've had years to understand who this ISIS threat, who this al Qaeda threat is, and the pattern is secondary device. If you're able to leave leave," he said.
A slogan often seen on airports signs and law enforcement personnel often repeat is, "See something, say something." Parker can't stress that enough that everyone should alert authorities to anything out of the ordinary or strange.
Universities have put students traveling abroad on high alert.
"We have reached out to all students studying abroad in that region," Jacksonville University said in a statement. "We encourage students to limit their travel in that area and continue to monitor updates from the Department of State and (the Overseas Security Advisory Council).
Beyond public scrutiny, security experts say the keys to effective screening are intelligence and constant change in procedures to keep terrorists guessing.
"Random is always good," said Brian Jenkins, a senior security analyst at the RAND Corp. Terrorists "don't like things that they can't predict. They want to know that a target is unprotected."
Jenkins added that visible presence of more police would be a deterrent and allow for quicker response to an attack.
In the U.S., airport security is complicated by the division of responsibilities. Typically, the Transportation Security Administration handles the screening of passengers and baggage but the airport or local police oversee security of terminals, parking lots and other public areas.
There are increased police patrols within terminals during times of heightened security but even then most passengers don't interact with police and aren't questioned until they reach the checkpoint. Airport operators and the TSA note that there are many layers to security, many which are not visible to the public.
The vulnerability of airports outside of the checkpoint has been the subject of studies, recommendations and has led to some changes in airport operations, said Richard Bloom, who teaches aviation security at Embry-Riddle Aeronautical University. But increased security requires more manpower and costs money.
Copyright 2016 by WJXT News4Jax. The Associated Press contributed to this report. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.