While it found no operational ties between al Qaeda and Iraq, the commission investigating the Sept. 11, 2001, attacks has concluded that Osama bin Laden's terrorist network had long-running contacts with Iraq's neighbor and historic foe, Iran.

Al Qaeda, the commission determined, may even have played a "yet unknown role" in aiding Hezbollah militants in the 1996 bombing of the Khobar Towers complex in Saudi Arabia, an attack the United States has long blamed solely on Hezbollah and its Iranian sponsors.

The notion that bin Laden may have had a hand in the Khobar bombing would mark a rare operational alliance between Sunni and Shiite Muslim groups that have historically been at odds. That possibility, largely overlooked in the furor of new revelations released by the commission last week, comes amid worsening relations between the United States and Iran, which announced on Thursday that it would resume building equipment necessary for a nuclear weapons program.

The Sept. 11 panel's findings on Iran have been eclipsed by the continuing political debate over Iraq, which the commission said had not developed a "collaborative relationship" with al Qaeda despite limited contacts in the 1990s. That appeared to conflict with previous characterizations made by President Bush, Vice President Cheney and other administration officials in their justifications for launching the war against Saddam Hussein.

In relation to Iran, commission investigators said intelligence "showed far greater potential for collaboration between Hezbollah and al Qaeda than many had previously thought." Iran is a primary sponsor of Hezbollah, or Party of God, the Lebanon-based anti-Israel group that has been designated a terrorist organization by the United States.

The commission's Republican chairman, former New Jersey governor Thomas H. Kean, also said in a television appearance last week that "there were a lot more active contacts, frankly, with Iran and with Pakistan than there were with Iraq."

But perhaps most startling was the commission's finding that bin Laden may have played a role in the Khobar attack. Although previous court filings and testimony indicated that al Qaeda and Iranian elements had contacts during the 1990s, U.S. authorities have not publicly linked bin Laden or his operatives to that strike, which killed 19 U.S. servicemen. A June 2001 indictment of 14 defendants in the case makes no mention of al Qaeda or bin Laden and lays the organizational blame for the attacks solely on Hezbollah and Iran.

Bruce Hoffman, a terrorism expert who heads the Washington office of Rand Corp., said that although bin Laden's then-fledgling group was an early suspect in the blasts, "the evidence kept pointing to an Iranian connection, so people tended to discount a bin Laden connection."

"What the commission report is raising is that the relationship might have been much tighter and was in fact operational and not just spiritual," Hoffman said.

U.S. officials who have worked on the Khobar case are more skeptical. A law enforcement source with knowledge of the case, who declined to be identified because of the ongoing criminal investigation, said authorities searched carefully for an al Qaeda connection but found no basis for it.

The broader notion of links between bin Laden's group and Hezbollah or hard-line elements in Iran's security forces has been a hot topic in U.S. law enforcement and intelligence circles for years. Many analysts have viewed such an alliance as dubious, largely because of ancient animosities between Shiite and Sunni Muslims. Several leaders of al Qaeda, a Sunni organization, have issued rabidly anti-Shiite proclamations.

Nonetheless, the United States previously compiled evidence of limited contacts between Iranian interests and al Qaeda. U.S. officials alleged that Iran was harboring al Qaeda militants who had fled neighboring Afghanistan after the U.S. invasion there.

Iran has denied that al Qaeda was operating from its territory, and announced earlier this year that it would put on trial a dozen suspected members of the terrorist group.

The original U.S. indictment of bin Laden, filed in 1998, said al Qaeda "forged alliances . . . with the government of Iran and its associated terrorist group Hezbollah for the purpose of working together against their perceived common enemies in the West, particularly the United States."

But the Sept. 11 commission's findings regarding Khobar Towers, if confirmed, would deepen the known relationship between al Qaeda, Iran and Hezbollah. A commission staff report issued June 16 said that in addition to evidence that the attack had been carried out by Saudi Hezbollah with assistance from Iran, "intelligence obtained shortly after the bombing . . . also supported suspicions of bin Laden's involvement.

"There were reports in the months preceding the attack that bin Laden was seeking to facilitate a shipment of explosives to Saudi Arabia. On the day of the attack, bin Laden was congratulated" by al Qaeda militants, the report says.

The report recounts some of the previously alleged contacts between al Qaeda and Iran or Hezbollah and concludes, "We have seen strong but indirect evidence that [bin Laden's] organization did in fact play some as yet unknown role in the Khobar attack."

The report also says that several years before the Khobar attack, "bin Laden's representatives and Iranian officials had discussed putting aside Shia-Sunni divisions to cooperate against the common enemy." A group of al Qaeda representatives then traveled to Iran and to Hezbollah training camps in Lebanon for "training in explosives, intelligence and security," the report says.

Bin Laden himself, the report added, "showed particular interest in Hezbollah's truck bombing tactics in Lebanon in 1983 that killed 241 U.S. Marines."

Flynt L. Leverett, a Middle East expert in the Clinton and Bush administrations who is now a Brookings Institution scholar, said active cooperation between al Qaeda and Iran "cannot be ruled out as wholly implausible."

"There are going to be serious structural limits to how much al Qaeda and Iran might cooperate," Leverett said. "Within those limits, though, there is some room for very tactical and self-serving cooperation between al Qaeda and some parts of Iranian intelligence." Leverett cited as an example the allegations that Iran had harbored al Qaeda operatives fleeing Afghanistan.

But Daniel Benjamin, a national security official in the Clinton administration, said he was "still skeptical" of any link between al Qaeda and Khobar, arguing that the evidence shows "that Saudi Hezbollah was very much a creature of some in Iran."

"I don't quite see the need that this operation had for assistance from al Qaeda," Benjamin said. "Second of all, my understanding of the larger relationship between Iran and al Qaeda suggests that while there were plenty of contacts, many more than there were with Iraq, it was never clear they developed a serious cooperative relationship."

Research editor Margot Williams contributed to this report.

U.S. troops at the site of the June 1996 bombing of the Khobar Towers complex in Dhahran, Saudi Arabia. The blast killed 19 U.S. service members.