The Brussels bombings have highlighted an inherent problem in airline security, say anti-terrorism experts: the crowds of waiting passengers caused by the need to check for weapons and bombs inadvertently creates its own terrorism target.

"Airport security is front-loaded as much as possible towards prevention of an event taking place on an airplane," said Bill Jenkins, a terrorism policy expert with the Rand Corporation. But making it impossible for terrorists to get on a plane doesn't prevent them from trying a different attack. They then look for other "mass casualty" targets, such as the airport terminal.

So, speeding up passenger processing by the Transportation Security Administration would reduce the danger by reducing the crowds.

Brussels was particularly vulnerable, Jenkins said, because it was a smaller, more compact airport than most. "Public spaces seem to be their killing fields now," he said.

Tuesday's attacks by three terrorists in the main terminal of the Brussels airport resulted in 11 deaths. There is no indication that the terrorists had any interest in boarding a plane. They simply made a direct attack on the crowds in the main area. Another 20 people were killed in an attack at nearly the same time on a nearby commuter rail system.

The Brussels airport attack is a scenario that Marshal McClain, president of the Los Angeles Airport Peace Officers Association, whose members guard LAX, thinks about constantly.

"Anything that lessens the crowds is a positive," he said. "The biggest issue at LAX is the traffic [at drop-off and pick-up areas]. You have got to get more people in and out of there quickly."

A 2004 Rand study on security risks at LAX said airports should limit crowds in unsecured areas as much as possible. "Overall airport efficiency, including the operations of [the airport], the airlines and TSA, is not enhanced by having people stand in line," it found.

Robert Poole, director of transportation policy at the Reason Foundation, a nonprofit free-market think tank, argues that the TSA, which is part of the Department of Homeland Security, has been lax in adapting to changes in terrorists' plots.

"TSA is still far too focused on fighting the last war — i.e., preventing a repeat of the 9/11 attack. That's why the vast majority of TSA's budget is focused on passenger and baggage screening, with basically no attention to the vulnerabilities to airports and their passengers that Rand has higlighted," Poole said.

A TSA spokesman said none of its security policy analysts were available for comment.

Attacks at airports, as opposed to airplane bombings or hijackings, are fairly common. The 2004 Rand study found that one-third of all international air travel-related attacks since 1980 — about 75 attacks causing 78 deaths — involved targeting passengers in the airport itself.

They have happened numerous times since the year 2000. In 2002, a lone Egyptian-born gunman opened fire at the LAX ticket counter for Israel's El Al airline, killing two and wounding four before he was shot by an Israeli security officer. In 2013, a Los Angeles man opened fire in LAX at the TSA screening area, killing a federal agent and injuring two other people.

One of the deadliest terrorist attacks in Russian history, a 2011 suicide bombing in the main terminal at Moscow's busiest airport, Domodedovo International, killed 37 people and injured 173 others. In 2007, Islamic radicals attempted to drive a truck filled with propane canisters through the main doors of Scotland's Glasgow International Airport. In that case, only the terrorists were injured.

It is not the size of the bomb that is the most important factor, experts note. The danger lies in the bomb's proximity to large groups of people. The Glasgow attack, for example, failed largely because the terrorists' truck struck security partitions before it could get to a more heavily occupied section of the airport. The 2013 Boston Marathon bombings, which killed three and injured 264, caused more damage because the terrorists dropped their bomb in the middle of a crowd of spectators.

One solution would be to limit vehicle traffic at airports and instead expand access through public transportation such as trams and rail lines, McClain said. That would limit the crowds to smaller areas and reduce the likelihood of a large truck bomb.

Jenkins said the TSA and airport officials are making efforts in speeding up passenger processing and reducing crowds. "Where that can be done, they are doing it," he said. But he added that there is only so much that can be done. Terrorists causing a plane crash is still a bigger concern than terminal attacks because a crash would likely result in a much greater likelihood of death. Speeding up security screenings increases the likelihood that an agent makes a mistake and a terrorist slips through.

The TSA even now has problems in that area. An internal investigation by DHS's Inspector General's Office last year found that undercover investigators were able to board planes at some of the nation's busiest airports while carrying mock weapons or explosives 95 percent of the time. The agency responded with an improvement plan calling for stricter examinations of passengers. Earlier this month, the TSA announced it would hire more officers in an effort to speed up processing.