Security experts worry that private planes could become al Qaeda's next weapon. An intelligence bulletin issued by the Department of Homeland Security and the FBI on Friday said:
"Violent extremists with knowledge of general aviation and access to small planes pose a significant potential threat to the homeland."
Although there is no information a specific attack is in the works, there are 228,000 general aviation planes at 4,000 airports across the nation -- too many to monitor.
The government is using signs that read "Warning: Pilots Report All Suspicious Activities" to keep pilots vigilant.
Intelligence experts say al Qaeda is no longer determined to pursue only massive 9/11-style attacks.
"They have sort of taken on this view of death by a thousand cuts, that if they try a lot of smaller attacks they are just as effective as the fear factor, so they really get more bang for their buck to do smaller attacks," said ABC News consultant and former FBI investigator Brad Garrett.
There are thousands of small planes in nearly every state. In College Park, Md., as soon as one plane lifts off it's in view of the Capitol, the Pentagon and about 10 miles away from the White House.
Last year, a man with a grudge against the federal government flew his plane into an Internal Revenue Service office in Austin, Texas.
The State Department has issued a worldwide travel alert and the threats go beyond small planes. Documents captured at Osama bin Laden's compound show that he was obsessed with plotting attacks around the 9/11 anniversary, including derailing a train over a bridge in the U.S.
Federal officials are also concerned about the threat of passengers implanted with body cavity bombs, deadly and nearly impossible to detect. And the threats go on.
"If seven or ten individuals came together and conducted a Mumbai, India, attack, you could go into a mall and kill more people potentially than at 9/11 in 15 minutes," said Garrett.
Then there is always the unknown. Al Qaeda, authorities warn, has shown an imaginative approach to terror.