The FBI (search) is warning that terrorists could be developing plans to use airplanes as weapons.
"Recent intelligence indicates that terrorists continue to develop plans to hijack aircraft and use them as weapons, despite enhanced worldwide airport security and increased vigilance on the part of passengers and crew members since the September 11 attacks,'' the FBI said in its weekly bulletin sent to law enforcement agencies throughout the country.
"Intelligence indicates terrorists are considering the use of improvised explosive devices (search) (IEDs) assembled onboard to hijack an aircraft or, alternatively, destroy it over heavily populated areas in the event of passenger or crew resistance," the bulletin said.
The FBI said that components of IEDs can be smuggled onto planes concealed in clothing or personal carry-on items like shampoo and medicine bottles, and then put together onboard.
"In many cases of suspicious passenger activity incidents have taken place in the aircraft's forward lavatory," the FBI noted. "It is conceivable terrorists may plan to use this private area to construct IED(s) in order to facilitate access to the cockpit, or position themselves in front of the passengers.''
Retired U.S. Air Force Col. Darrel Whitcomb said it's obvious there is recent intelligence prompting the FBI to issue this bulletin.
"We are once again very aware of the fact that the enemy realizes how important aviation is to us as a nation and once again they're going to try to use it against us," Whitcomb told Fox News. "That's what they did on 9/11 -- they took those beautiful airplanes that we were comfortable with and turned them against us."
The bureau also makes reference in the two-page bulletin to the college student who recently smuggled hoax explosives made of modeling clay onboard several airplanes.
"Several other incidents involving vandalism of aircraft lavatories and possible attempts to access the cockpit bulkhead have also been reported," the FBI said. "Although these incidents do not appear to be terrorist related, the widespread media coverage of these incidents highlights screening vulnerabilities at specific airports that could be exploited by terrorists."
The FBI told law enforcement officials and security personnel to "remain vigilant for attempts, possibly involving one or more individuals, to smuggle IED components onboard aircraft."
Fox News' Anna Stolley contributed to this report.