An audio recording supposedly of Osama Bin Laden denies Zacarias Moussaoui, convicted in the US over the 9/11 attacks, was involved in the operation.
In an April recording, Bin Laden said the West was at war with Islam
The message said to be from the al-Qaeda leader came from a five-minute audio file on an Islamist website.
The man said he had personally assigned 19 attackers and "brother Zacarias" was not one of them.
Earlier this month, a US court jailed Moussaoui for life without parole for his role in the attacks.
"I am the one in charge of the 19 brothers and I never assigned brother Zacarias to be with them in that mission," the voice said, in a reference to the 19 hijackers of 11 September 2001.
"Since Zacarias Moussaoui was still learning how to fly, he wasn't No 20 in the group, as your government claimed," he said.
Moussaoui confessed because of pressure caused by over four years in prison, he said.
Moussaoui, a French citizen of Moroccan descent, pleaded guilty in April 2005 to six charges of conspiracy over the hijacking attacks on New York and Washington.
The man in the recording also said that none of the Guantanamo Bay detainees were connected to the attacks.
An unidentified US counterterrorism official told the Associated Press news agency that the US was aware of the message and there was no reason to doubt its authenticity.
Moussaoui has been sentenced to life without parole
If verified, the recording would be the third statement by Bin Laden this year.
On 23 April, Arab satellite TV al-Jazeera broadcast a tape purportedly from Bin Laden in which he said the West was at war with Islam. In a tape released in a similar way in January, Bin Laden warned of further attacks on the US.
The new audio recording would also be one of the al-Qaeda leader's clearest statements on his operational involvement in the 11 September 2001 attacks, correspondents say.
But the importance of this recording does not lie in the veracity or otherwise of its claims, says the BBC's Adam Brookes.
It lies in the fact that it is a reminder both to the Americans and to Bin Laden's sympathisers worldwide that he remains alive, at large and a political player, our correspondent says.