Arrests last month in Barcelona underline the fear. More than a dozen trained suicide bombers - many of them European citizens just back from Pakistan - were taken down as they prepared to launch attacks against transit systems in Spain and four neighboring countries.
"They've realized that if they want to operate successfully, they need to have people who look like us, act like us, and are very difficult to find," said Philip Mudd, a top counter-terrorism official at the FBI.
Is al Qaeda actively looking for American or Western recruits?
"Whether it's Americans or whether it's people who can launch from Europe, because they have visas or passports that allow them to travel to the United States, they are looking for people who can operate in the West," Mudd said.
In an exclusive interview with CBS News, Mudd said intelligence has tracked Western recruits to small training centers in Pakistan's rugged tribal region.
Many of the recruits are European citizens of Pakistani descent raised in London, Berlin, and Madrid - but drawn to the radical Islamic movement. And there have been a few Americans. Among them is convicted terrorist Jose Padilla, who grew up in Chicago, and Adam Gadahn from California, who is now a leading voice in al Qaeda's propaganda.
Unlike the large-military style camps al Qaeda used in Afghanistan before 9/11, the new training is being done in small groups, specifically tailored to Western recruits plotting strikes against Europe and the U.S.
"There is a different kind of person who is going to come to Western Europe, and they don't necessarily need paramilitary training; you might simply be talking about how do you build a small device that you could put in a back pack so you could take it on a train," Mudd said.
The transit bombers who hit London in 2005 were trained in Pakistan. So, too, were al Qaeda recruits arrested a year later in a plot to blow up passenger jets over the Atlantic.
But, officials warn the U.S. remains al Qaeda's top target.
"The purpose of the recruitment is to train people in weapons of mass destruction and then get them back into the United States," said Director of National Intelligence Mike McConnell.
Understanding al Qaeda's new tactics offers a first line of defense. But the real security challenge, at the airport or the border, may be finding the terrorists - if they look like us.