President George W Bush has said the US is exploring whether Iran had a role in the 11 September 2001 terror attacks.
The CIA says eight hijackers passed through Iran
"We're digging into the facts to see if there was one," Mr Bush told reporters at the White House.
His comments come after the CIA's acting chief said some of the hijackers passed through Iran, but there was no evidence Iran was officially involved.
It comes ahead of the publication this week of a final report by a US commission of inquiry into the attacks.
"We will continue to look and see if the Iranians were involved," said Mr Bush.
"As to direct connections with 11 September, we're digging into the facts to determine if there was one."
On Sunday, the CIA's acting director, John McLaughlin, told US television that the CIA had known for some time that eight of the hijackers travelled through Iran.
"We have ample evidence of people being able to move back and forth across that terrain," Mr McLaughlin told Fox News Sunday.
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."
Iran acknowledges some of the hijackers may have crossed its borders, but says they would have done so illegally.
US media has said the final report by the 9/11 inquiry 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 closer scrutiny by US border officials.
Washington has long regarded Iran as a state sponsor of terrorism, branded part of an "axis of evil" by President Bush.
Mr Bush also accused Iran of "harbouring al-Qaeda leadership" and demanded al-Qaeda operatives in Iran be handed over to their native countries.
Iran said last weekend it had taken action to dismantle al-Qaeda's network in the country, and dismissed the latest US criticism.