WASHINGTON (AP) — One day after the 10th anniversary of the Sept. 11 attacks, a former Obama administration official predicts no letup in the al-Qaida threat to the U.S.

Michael Leiter, the just-retired chief of the National Counterterrorism Center, said Monday that al-Qaida still intends to attack by any means possible, including unconventional weapons like biological agents.

Leiter told an audience at the Woodrow Wilson Center that al-Qaida's main leadership will be eclipsed over the next decade by dangerous offshoots like Yemen's al-Qaida of the Arabian Peninsula.

He referred to the covert CIA drone war against al-Qaida in Pakistan as "air strikes," calling them the single most effective tool against the group.

But he said it's likely al-Qaida will eventually manage to breach U.S. counterterrorist defenses, and called for Americans to be resilient.

(Copyright 2011, The Associated Press)