Authorities say there is no specific or credible terrorist threat for the 10-year anniversary of the attacks on the World Trade Center and Pentagon. But they have stepped up national security 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.
The alert, issued ahead of the summer's last busy travel weekend, said terrorists have considered renting private planes and loading them with explosives.
"Al-Qaeda and its affiliates have maintained an interest in obtaining aviation training, particularly on small aircraft, and in recruiting Western individuals for training in Europe or the United States, although we do not have current, credible information or intelligence of an imminent attack being planned," according to the bulletin obtained by The Associated Press.
The bulletin also says al-Qaeda would like to use sympathetic Westerners to get flight training, then get them to become flight instructors.
Matthew Chandler, a spokesman for the Department of Homeland Security, described the bulletin as routine.
"We shared this information with our partners to highlight the need for continued awareness and vigilance," he said.
Aviation security is much tighter than it was a decade ago, but al-Qaeda remains keenly interested in launching attacks on airplanes, believing large attacks with high body counts are more likely to grab headlines.