From the BBC:

Al-Qaeda's deputy leader, Ayman al-Zawahiri, has blamed Iran for spreading the theory that Israel was behind the 11 September 2001 attacks.

In an audio tape posted on the internet, Zawahiri insisted al-Qaeda had carried out the attacks on the US.

He accused Iran, and its Hezbollah allies, of trying to discredit Osama Bin Laden's network.

Correspondents say the comments underline al-Qaeda's increasing public hostility towards Iran. ...

The purpose of this lie is clear - [to suggest] that there are no heroes among the Sunnis who can hurt America as no-one else did in history," he said.

"Iranian media snapped up this lie and repeated it."