Why it's tough to secure airports, transit after Brussels

The Brussels attacks represent a new terrorist strategy that will be challenging for law enforcement to tackle, a former TSA assistant administrator said Thursday.

The deadly bombings in the departure hall at Brussels' main airport, for which ISIS claimed responsibility, marked a shift from attempting to get prohibited items through checkpoints and onto airplanes to targeting a lower-security area of a public space, Chad Wolf told CNBC's "Squawk Box."

The Transportation Security Administration's primary response will be to temporarily increase the law enforcement presence in these vulnerable areas, he said.

"Measures like that of a little bit of a random nature is really what the right response is going to be," he said. "Outside of that, you're going to continue to create more lines, more choke points if you try to institute more aggressive measures."

The United States would face significant costs if it were to adopt airport security measures like those in Israel, where agents approach travelers in the departure hall and question them, Wolf said. He noted that there are hundreds more airports in the United States.

The choices are limited, but TSA has experimented with behavioral detection to try to pick bad actors out of a crowd, but such programs face criticism about profiling, Wolf said.

Behavioral screening could help to protect metro systems and subways, as well, former Assistant Homeland Security Secretary Robert Liscouski told "Squawk Box" on Tuesday.

A second deadly bomb attack in Brussels targeted a central metro station.

Firearms and bombs are the most ubiquitous threats in urban areas, and the only way to detect them is through physical search, Liscouski said. However, those searches would have to be carried out in a systematic way through behavioral analysis, he added.

Those measures did not necessarily mean racially profiling commuters, he said.

"There are things that people do that you can pick up that are cues that are indicative of the fact that they may not necessarily there for the right reasons," he said.