Terrorism expert Dr. Jeff Addicott says it’s not surprising that some airlines have reported what might be “dry runs” by suspects planning an attack.  The most recent was reported last month on a flight from Reagan National Airport to Orlando.

“That's one of the things that law enforcement looks for,” Addicott says. “Terrorism is not a random act.  It's directed, it's thought out and therefore that requires a practice, or a training mission.”

Addicott says passengers can help spot terrorists who might be involved in activities leading up to another 9-11.

“The funding has kind of dried up and so the vast majority of flights do not have air marshals on them,” he says. “The first line of defense is really passengers.”

If you see something out of the norm, you should notify the flight crew.