The FBI and the Department of Homeland Security have issued a nationwide warning about al Qaeda threats against small planes.

The warning comes just days ahead of the tenth anniversary of the 2001 terror attacks on New York and Washington.

Authorities say there is no specific or credible terrorist threat, but they have stepped up security nationwide as a precaution.

According to a five-page law enforcement bulletin issued Friday, as recently as early this year, al Qaeda was considering ways to attack airplanes.

Officials warn the terror group is not only plotting attacks on a large scale, but they are also focused on smaller, less complicated ones.

"These planes are small. You might not be able to put some huge device aboard them. But the other side of that is if you were to put some biological or chemical agent in one of them, clearly that could cause some major damage depending on your target," former FBI agent Brad Garrett said.

It is estimated there are more than 220,000 small planes in the U.S.