While U.S. officials say publicly there is no specific threat of a terror attack, behind the scenes law enforcement officials tell ABC News there are plans for a major security surge at airports and transportation hubs in advance of next week's anniversary of Osama bin Laden's death.
The precautions are based on intelligence reports that al Qaeda is determined to avenge the death of bin Laden, killed by Navy SEALs last May, with a focus on aviation targets.
Of greatest concern to U.S. officials is al Qaeda's Yemeni affiliate, al Qaeda in the Arabian Peninsula (AQAP), and its master bombmaker, Ibrahim al-Asiri, who has survived repeated U.S. efforts to kill him.
It was al-Asiri, according to U.S. officials, who designed the so-called "underwear bomb" worn by Umar Farouk Abdulmutallab, who tried to bring down Northwest flight 253 from Amsterdam to Detroit on Christmas Day 2009. Abdulmutallab got the bomb past airport security but failed to detonate it successfully aboard the plane.
Officials say al-Asiri also designed the bombs hidden in printers that were shipped from Yemen to Chicago. The bombs were intercepted in Dubai and the U.K. after they'd been placed aboard cargo planes.
In a joint intelligence bulletin issued overnight, the FBI and the Department of Homeland Security said the Yemen group "intends to advance plots along multiple fronts, including renewed efforts to target Western aviation."
"It doesn't take a great number of people to do the kind of attack that we had on September 11," said Richard Clarke, an ABC News consultant and former White House counterterrorism official. "That was less than two dozen people and it's clear that they have that number available in places like Yemen today."
Threats of a revenge attack have been monitored by the U.S. ever since last year's raid on bin Laden's compound in Abbottabad, Pakistan.
Among the papers found in his home were repeated references to the importance of attacks timed to coincide with anniversaries.
Said Clarke, "I think the major issue for al Qaeda is to do something, to prove that they're still alive, to do some fairly major event or series of attacks that prove that they're not down, they're not out."
As a result, American law enforcement and White House officials say travelers at airports in the U.S. and Europe should expect to see enhanced security over the next several days.