National Counterterrorism Center Director Michael Leiter says the failed Christmas Day attempt to bring down a U.S. airliner is the starkest reminder of that threat.
Leiter said in a statement Saturday that officials "know with absolute certainty" that al-Qaida and others are trying to refine their methods.
The center 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.
