Iran is harboring members of al Qaeda, and the United States is investigating whether the Iranian government had a role in the September 11, 2001, attacks, President Bush said Monday.
Bush said the CIA has found no sign of a direct connection between Iran and the suicide hijackings that killed nearly 3,000 people.
"We will continue to look and see if the Iranians were involved," he said.
"I have long expressed my concerns about Iran. After all, it is a totalitarian society where free people are not allowed to exercise their rights as human beings."
The bipartisan, independent commission investigating the 9/11 attacks is expected to issue its final report this week.
The commission has found that eight to 10 of the hijackers passed through Iran between October 2000 and February 2001, Time magazine reported this week.
The magazine said that commission investigators have found that Iran had a history of allowing al Qaeda members to enter and leave the country across the Afghan border.
But the report does not offer evidence that Tehran was aware of the plans for the 9/11 attack.
On Monday, Bush accused Iran of harboring suspected al Qaeda members and developing nuclear weapons. If the country's Islamic government is to improve ties with Washington, the president said, it must hand over any al Qaeda members to their home countries, abandon its suspected nuclear weapons program and end its support of Islamic militant groups such as Hezbollah, which the United States considers to be a terrorist organization.
"As to direct connections to September 11, we're digging into the facts to determine if there was one," Bush said.
Iran has said it has al Qaeda members in its custody and will put them on trial. It denies trying to develop a nuclear bomb, saying its nuclear program is aimed at producing electrical power.
But the International Atomic Energy Agency, the U.N. nuclear watchdog, has rebuked Iran for not cooperating with the international community.
Bush branded Iran part of an "axis of evil" in his 2002 State of the Union speech, along with Iraq and North Korea.
The United States led an invasion into Iraq a year later after accusing Baghdad of harboring terrorists and concealing weapons of mass destruction from U.N. inspectors. Nearly 140,000 U.S. troops remain in Iraq after the establishment of an interim government.