A top counterterrorism official is warning that al-Qaida and other extremists are working to test US defences and launch an attack on American soil.
National Counterterrorism Centre Director Michael Leiter says the failed Christmas Day attempt to bring down a US airliner is the starkest reminder of that threat.
Leiter said in a statement yesterday that officials "know with absolute certainty" that al-Qaida and others are trying to refine their methods.
The centre is part of the Office of the Director of National Intelligence. It draws experts from the CIA, FBI, Pentagon and other agencies who try to ensure that clues about potential attacks are not missed.
Terrorism expert Harvey Kushner of Long Island University said a significant concern is that many US airports don't currently have the necessary technology to protect flights from the type of explosives attack attempted on Christmas.
The suspect, Umar Farouk Abdulmutallab, apparently assembled an explosive device, including 80 grams of Pentrite, or PETN, in the aircraft toilet of a Detroit-bound Northwest flight, then planned to detonate it with a syringe of chemicals. Passengers intervened, and the plan failed. "What's disturbing about this is we're almost nine years after 9/11 and billions of dollars have been spent and we don't have in place a system which can make us safe in the air," Kushner, chair of the school's Criminal Justice Department, told The Associated Press.