Al Qaeda poised to try major attack in United States within 3-6 months, intelligence chiefs warn

"An attempted attack ... is certain, I would say," Director of National Intelligence Dennis Blair told a Senate hearing on terror threats.

Al Qaeda still wants to hit the U.S. with a massive assault, but drone strikes against its leadership in Pakistan's tribal turf have hurt its ability to launch terror attacks on the scale of 9/11, which killed almost 3,000 people.

"Complex, multiple cell-based attacks could still occur, [but] we are making them very difficult to pull off," Blair said.

"The biggest threat is not so much that we face an attack like 9/11. It is that Al Qaeda is adapting its methods in ways that oftentimes make it difficult to detect," CIA Director Leon Panetta said.

Blair also disclosed publicly for the first time that Al Qaeda has eyed destroying the D.C. Metro subway system.