The CIA's acting director says eight of the 9/11 hijackers passed through Iran, but there is no evidence Tehran was connected with the attacks in 2001.
The US says it was Iranian policy to allow al-Qaeda members to pass
John McLaughlin told US TV the CIA had known for some time that eight of the hijackers travelled through Iran.
He was speaking ahead of publication of a final report by an inquiry into the attacks on Washington and New York.
Iran acknowledges some of the hijackers may have crossed its borders, but says they would have done so illegally.
"This is not surprising to us. I think the count is about eight of the hijackers were able to pass through Iran at some point," Mr McLaughlin told Fox News Sunday.
"We have ample evidence of people being able to move back and forth across that terrain."
But he added: "However, I would stop there and say we have no evidence that there is some sort of official sanction by the government of Iran for this activity.
"We have no evidence that there is some sort of official connection between Iran and 9/11."
The US media has said the final report by the 9/11 inquiry, due out this week, will say that from October 2000, it was official Iranian policy to allow al-Qaeda members into and out of the country, without having their passports stamped.
An Iranian stamp would have made the operatives subject to more scrutiny by US border officials.
Washington has long regarded Iran as a state sponsor of terrorism; President George W Bush famously included it in his "axis of evil".
The US has accused Iran of providing a safe haven to many al-Qaeda members since the war in neighbouring Afghanistan.
Iran said this weekend it had taken action to dismantle al-Qaeda's network in the country, and dismissed the latest US criticism.
"The more we approach the [US] presidential elections,
we will witness more of such news fabrications," said
Foreign Ministry spokesman Hamid Reza Asefi.