U.S. intelligence officials, already focused on Iran's potential for building nuclear weapons, are struggling to solve a more immediate mystery: the murky relationship between the new Tehran leadership and the contingent of Al Qaeda leaders residing in the country.
Some officials, citing evidence from highly classified satellite feeds and electronic eavesdropping, believe the Iranian regime is playing host to much of Al Qaeda's remaining brain trust and allowing the senior operatives freedom to communicate and help plan the terrorist network's operations.
And they suggest that recently elected President Mahmoud Ahmadinejad may be forging an alliance with Al Qaeda operatives as a way to expand Iran's influence or, at a minimum, that he is looking the other way as Al Qaeda leaders in his country collaborate with their counterparts elsewhere.
"Iran is becoming more and more radicalized and more willing to turn a blind eye to the Al Qaeda presence there," a U.S. counter-terrorism official said.
The accusations from U.S. officials about Iranian nuclear ambitions and ties to Al Qaeda echo charges that Bush administration figures made about Iraq in the run-up to the U.S.-led invasion three years ago.
Those charges about Iraq have been discredited. And in the case of Iran, some intelligence officials and analysts are unconvinced that Al Qaeda operatives are being allowed to plot terrorist acts. If anything, they suggest, the escalating tensions between Shiite and Sunni Muslims in Iraq would logically cause Iran's Shiite government to crack down on Al Qaeda, whose Sunni leadership has denounced Shiites as infidels.
A U.S. intelligence official said he did not see any relaxation in Iran's restrictions on Al Qaeda members.
"I'm not getting the sense that these people are free to roam, free to plot," the official said.
Still, the official acknowledged that the relationship between Tehran and Al Qaeda officials within Iran was largely unknown to U.S. and allied intelligence, especially since Ahmadinejad's election last summer.
To some U.S. intelligence officials, what worries them most is what they don't know.
"I don't need to exaggerate the difficulty in determining what these people are up to at any given moment," the intelligence official said.
The U.S. counter-terrorism official was more blunt. "We don't have any intelligence going on in Iran. No people on the ground," he said. "It blows me away the lack of intelligence that's out there."
U.S., European and Arab intelligence officials spoke on condition of anonymity because they were not authorized to discuss the issues publicly.
Ties between Iran and Al Qaeda were highlighted by the Sept. 11 commission, which disclosed a wealth of details about such connections in its final report. The commission said Iran and Al Qaeda had worked together sporadically throughout the 1990s, trading secrets, including some related to making explosives.
Iranian representatives to the United Nations did not return repeated phone calls seeking comment.
In November, the State Department's third-ranking official, Undersecretary R. Nicholas Burns, said the U.S. believed "that some Al Qaeda members and those from like-minded extremist groups continue to use Iran as a safe haven and as a hub to facilitate their operations."
A year ago, Iranian delegates to a global counter-terrorism conference circulated a document describing Iran as "a major victim of terrorism." The document blamed links between drug trafficking and terrorism for "thousands of security problems," especially along Iran's eastern border with Afghanistan and Pakistan.
Al Qaeda operatives and family members have lived in Iran for years, many since late 2001, when they fled the U.S.-led bombing of Afghanistan. Many other Al Qaeda figures fled to Pakistan -- a U.S. ally -- and are believed to be there still.
Four months ago, Iran declared that no Al Qaeda members remained in the country, but U.S. officials reject the claim. At other times, Iranian officials said that Al Qaeda members were kept under house arrest and their activities monitored.
In Tehran, analysts said American officials were misreading Iran's intentions. The fact that the government has not heeded U.S. demands to turn over Al Qaeda suspects should come as no surprise given the state of relations between the two countries, said Nasser Hadian, a political analyst at Tehran University.
"They won't. Why should they" without receiving something in return? he said.
Some of the suspects have been indicted in the United States in connection with terrorist attacks, including the 1998 bombings of two U.S. embassies in East Africa, but Iran has refused to extradite them.
Among them is Saif Adel, believed to be one of the highest-ranking members of Al Qaeda, behind Osama bin Laden and Ayman Zawahiri. Whatever restrictions might be placed on the network's activities within Iran, Adel -- who has a $5-million U.S. bounty on his head -- was able last year to post a lengthy dispatch about Al Qaeda activities in Iran and Iraq that was widely circulated on the Internet. U.S. intelligence officials consider the posting authentic.
In the dispatch, Adel said he had used hide-outs in Iran to plot with Abu Musab Zarqawi to make Iraq the new battleground in the group's war against the United States. Iran had detained many of Zarqawi's men, Adel wrote, but they ultimately slipped into Iraq and began attacking U.S. forces.
U.S. officials say intelligence suggests that Al Qaeda operatives have engaged in at least some terrorist planning from Iran, including Adel's alleged orchestration of suicide bombings in Saudi Arabia in May 2003 and the masterminding of several attacks in Europe.
For several years, the U.S. counter-terrorism official said, satellite feeds have helped officials monitor some of the day-to-day activities and movements of Adel and other senior Al Qaeda operatives in Iran. The intelligence suggests that the Al Qaeda leaders have been monitored by Iranian authorities but could move and communicate somewhat, the official said.
U.S. officials also said that other senior Al Qaeda figures -- including Zarqawi, now the group's point man in Iraq -- had moved in and out of Iran with the possible knowledge or complicity of Iranian officials.
The Al Qaeda members in Iran include three of Bin Laden's sons. Some of his wives and other relatives are suspected of being there as well, as is Al Qaeda spokesman Sulaiman abu Ghaith, U.S. officials say.
Of special concern, they said, is the number of Al Qaeda operatives in Iran who are of Egyptian descent and loyal to Zawahiri, the Cairo-born physician who merged his Egyptian Islamic Jihad with Al Qaeda in the years before the Sept. 11 attacks.
Adel is a former Egyptian police official. In addition, U.S. officials confirmed intelligence showing that three other Al Qaeda operatives with Egyptian roots -- Abdallah Mohammed Rajab Masri, also known as Abu Khayer; Abdel Aziz Masri; and Abu Mohamed Masri -- are in Iran. Authorities believe them to be, respectively, the head of Al Qaeda's leadership council, a biological weapons expert who heads the network's effort to develop weapons of mass destruction; and its top explosives expert and training camp chief.
The U.S. counter-terrorism official said the Egyptians' presence was troubling because Tehran for more than a decade has supported Egypt's two largest militant groups -- Egyptian Islamic Jihad and Gamaa al Islamiya -- in their violent campaign to topple the Cairo government.
Though the Sunni-Shiite divide has prompted Tehran in the past to say it had "no affinity" with Al Qaeda, U.S. officials believe there is a history of cooperation between Iran and some Sunni militant groups, including Al Qaeda. Iran nurtures such ties, they say, to enhance its regional influence and punish Arab political foes through intimidation and violence.
Bin Laden sent Adel and others to Iran and Lebanon in the early 1990s to learn bomb making from Iranian intelligence and Hezbollah, the Iran-affiliated militant group, U.S. officials say. They fear he and other Egyptians may still have ties with Iran's military and intelligence services.
The Sept. 11 commission concluded that Iran had harbored Al Qaeda operatives wanted in the U.S. embassy bombings in East Africa and other terrorist attacks.
It quoted one top Al Qaeda official as saying Iran had made a "concerted effort to strengthen relations with Al Qaeda" after the 2000 attack on the U.S. warship Cole in Yemen.
Imprisoned top Al Qaeda operatives also have told U.S. officials that Iran let Islamic militants traveling to and from Afghanistan and Pakistan pass freely across its borders without passport stamps -- including at least eight of the 19 future Sept. 11 hijackers, the nowdisbanded commission said.
The panel strongly urged the Bush administration and Congress to investigate the ties between Iran and Al Qaeda. Recently, commission member Timothy Roemer said in an interview that Washington still had not adequately addressed those ties.
U.S. and allied intelligence agencies say that, more recently, they have picked up indications of closer cooperation. The intelligence includes European wiretaps of militants discussing how Iranian officials would help them or look the other way.
U.S. officials fear Ahmadinejad may be strengthening ties with Al Qaeda with the help of Iranian intelligence and military agencies, particularly the Revolutionary Guards.
The intelligence official and others noted that Ahmadinejad himself rose through the ranks of the guards, an elite military unit. U.S. government officials have accused the guards of financing and orchestrating terrorist acts in the region by groups including Hezbollah, which is suspected of blowing up U.S. military facilities and embassies in the 1980s and killing hundreds of Americans.
Rep. Brad Sherman of Sherman Oaks, the ranking Democrat on the House International Relations subcommittee on terrorism and nuclear proliferation, who receives classified briefings on Iran, said U.S. intelligence indicated that Tehran was engaged in some kind of collaboration with Al Qaeda leaders.
"The cooperation is substantial," Sherman said. "Key operatives of the most successful terrorist organization in history are spending their time in the No. 1 state sponsor of terrorism.... That is of massive concern."
U.S. officials fear that an Iranian hard-line faction or even a rogue official could conspire with Al Qaeda or provide access to the country's military arsenal.
Despite the mutual antipathy between Sunnis and Shiites, some U.S. officials argue that the Iranian regime and Al Qaeda share a common enemy -- the United States -- and that both oppose the establishment of a pro-Western democracy in Iraq.
John D. Negroponte, the director of national intelligence, told Congress on Feb. 2 that Iran was engaged in a broad campaign "to disrupt the operations and reinforcement of United States forces based in the region, potentially intimidating regional allies into withholding support for United States policy toward Iran and raising the costs of our regional presence" for the U.S. and its allies.