Terror operatives could attempt to hijack commercial, charter or tourist helicopter flights and use them in suicide attacks against buildings, parades or sporting events, said one of the intelligence bulletins obtained Monday by The Associated Press.
Helicopters and limousines could both help terrorists get closer to targeted buildings, the FBI warnings said. The warnings were sent Friday night by the FBI and the Homeland Security Department to 18,000 state and local law enforcement agencies, other government officials and private groups.
Although the Bush administration's recent heightened terror alert was confined to financial institutions in New York, Newark, N.J., and Washington, the FBI said al Qaeda was interested in using helicopters to attack "any densely populated area of symbolic, economic or financial importance" in the United States.
The other bulletin warns that al Qaeda has frequently used rented cars and trucks for bomb attacks in the past — including the 1993 attack on the World Trade Center — and may do so again.
Limousines, the FBI said, could be especially useful to terrorists because they are larger than regular cars and might draw less suspicion than trucks.
"Limousines often convey an impression of authority or prestige, which may facilitate their access to specific locations in a building or a facility denied to the general public," the FBI bulletin said.
The FBI says it has no credible, specific evidence about the method, timing or location of any al Qaeda attack inside the United States. But the bulletins come amid a steady stream of intelligence indicating that the terror group intends to strike in the months leading to the Nov. 2 presidential election.
The FBI bulletins were also issued after arrests in Pakistan and Britain of several key al Qaeda operatives and the seizure of computers, computer files and other evidence. U.S. officials say they are hopeful that this lode of information might be critical to disruption of plots that may be in the final stages.
Three laptop computers and 51 discs seized in the July 24 raid in Pakistan, Time magazine reported, quoting an unidentified U.S. law enforcement official.
Time magazine correspondent Doug Waller said on CBS News' The Early Show the very-detailed surveillance reports were "breath-taking" in their sophistication.
"It has details on the load-bearing capabilities of the (Citigroup building in Manhattan) and how it would be able to withstand any particular type of explosion," he said.
On the helicopter leads, the FBI says it has information that indicates al Qaeda operatives have considered using helicopters packed with explosives in an unspecified attack.
Helicopters might also be used to spread chemical or biological agents in the ventilation systems of high-rise buildings, the FBI bulletin said.
"Terrorists may view helicopters as an attractive weapon due to their maneuverability and non-threatening appearance when flying at low altitudes," the FBI said.
"Clearly, using small helicopters would be less than 1/100th of the effect of flying a 757 into a building," said CBS News Terrorism Consultant Randall Larson. "It might be more of just a brain-storming session with al Qaeda about ways they can do things."
Police around the country are being urged by the FBI to pay extra attention to commercial and private helicopter operations and schools. The businesses also are warned to be alert to anyone with undue interest in helicopter payloads and security procedures and to set up screening and identification procedures for passengers and cargo.
Passenger screening prior to liftoff may be required as early as this week, although some helicopter operators say they've beefed up security on their own anyway.
Metal detectors also are suggested for screening of helicopter passengers to discourage them from "carrying weapons or dangerous items on board charter or tour helicopters," the FBI said.
Vigilance also is being urged for car and truck rental companies, particularly for any activities suggesting that customers may be involved in preparations for an upcoming attack. These include: