WASHINGTON -- 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-Qaida terror network leaders residing in the country.

Some officials, citing evidence from highly classified satellite feeds and electronic eavesdropping, believe that the Iranian regime is hosting much of al-Qaida's remaining brain trust and allowing the senior operatives freedom to communicate and help plan the terrorist network's operations.

And they suggest that new President Mahmoud Ahmadinejad may be forging an alliance with al-Qaida operatives as a way to expand Iran's influence or, at minimum, that he is looking the other way as al-Qaida leaders in Iran collaborate with their counterparts elsewhere.

"Iran is becoming more and more radicalized and more willing to turn a blind eye to the al-Qaida presence there," said one U.S. counterterrorism official.

The accusations from U.S. officials about Iranian nuclear ambitions and ties to al-Qaida echo charges that administration figures made about Iraq in the run-up to the U.S. invasion three years ago.

Those charges about Iraq have been largely discredited. And in the case of Iran, some intelligence officials and analysts are unconvinced. If anything, they suggest, escalating tensions between Shiites and Sunni Arabs in Iraq would logically cause Iran's Shiite government to crack down on al-Qaida, whose Sunni leadership has denounced Shiites as infidels.

A U.S. intelligence official says he does not see any relaxation in Iranian restrictions on al-Qaida members. "I'm not getting the sense that these people are free to roam, free to plot," the intelligence official said.

But even that official conceded that the relationship between Tehran and al-Qaida officials within Iran is largely unknown to U.S. and allied intelligence, especially since Mr. Ahmadinejad's election last summer.

To some U.S. intelligence officials, what they don't know is the most worrisome of all. "I don't need to exaggerate the difficulty in determining what these people are up to at any given moment," said the official.

The U.S. counterterrorism 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 are not authorized to discuss such matters publicly.

Ties between Iran and al-Qaida were highlighted by the Sept. 11 commission, which disclosed a wealth of detail about such connections in its final report. The commission said Iran and al-Qaida had worked together sporadically throughout the 1990s, trading secrets, including how to make explosives.

Iranian representatives to the United Nations did not return repeated calls seeking comment. Three months ago Iran declared there were no more al-Qaida members in the country. U.S. officials reject that claim.

In November, the State Department's third-ranking official, Undersecretary R. Nicholas Burns, said the United States believes "that some al-Qaida 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 counterterrorism 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-Qaida operatives and family members have lived in Iran for years, many since late 2001 when they fled the U.S. bombing of Afghanistan. Many other al-Qaida figures fled to Pakistan -- a U.S. ally -- and are believed to still be there. Iranian government officials have said the al-Qaida members in their country have been kept under house arrest and that their activities have been monitored.

In Tehran, analysts said, U.S. officials are misreading Iran's intentions. The fact that the Iranian government has not turned over al-Qaida suspects to the United States should be no surprise given the state of relations between the two countries, said Nasser Hadian, a political analyst at Tehran University.

Some of the al-Qaida members have been indicted in the United States for terrorist attacks, including the 1998 bombings of two U.S. embassies in Africa, but Iran has refused to extradite them.

Among them is Saif al-Adel, believed to be one of the highest-ranking members of al-Qaida's hierarchy, behind Osama bin Laden and his deputy, Ayman al-Zawahri. Whatever restrictions might be placed on al-Qaida activities within Iran, Mr. Adel -- whose capture would carry a $5 million U.S. reward -- was able last year to post a lengthy dispatch about al-Qaida activities in Iran and Iraq that was widely circulated on the Internet. U.S. intelligence officials believe the posting was authentic.

In the dispatch, Mr. Adel said he had used hideouts in Iran to plot with Abu Musab al-Zarqawi to make Iraq the new battleground in the group's war against the United States. Iran had detained many of Mr. Zarqawi's men, Mr. Adel wrote, but they ultimately slipped into Iraq and began attacking U.S. forces.

For several years, the U.S. counterterrorism official said, satellite feeds have helped officials monitor some day-to-day activities and movements of Mr. Adel and other senior al-Qaida operatives in Iran. The intelligence suggests that the al-Qaida leaders have been monitored by Iranian authorities but could move and communicate somewhat, the official said.

U.S. officials also said other senior al-Qaida figures -- including Mr. Zarqawi, now the group's point man in Iraq -- have transited in and out of Iran with the possible knowledge or complicity of Iranian officials.

The al-Qaida members in Iran include three of Osama bin Laden's sons, including two who are considered his heirs apparent -- Saad and Hamza. Some of bin Laden's wives and other family members are suspected of being in Iran as well, as well as al-Qaida spokesman Suleiman Abu Ghaith, U.S. officials say.

Of special concern, the official said, is the number of al-Qaida operatives in Iran who are of Egyptian descent and loyal to Mr. Zawahri, the Cairo-born physician who merged his Egyptian Islamic Jihad with al-Qaida in the years before the Sept. 11 attacks.

Mr. Adel is a former Egyptian police official. In addition, U.S. officials confirmed intelligence showing three other al-Qaida operatives with Egyptian roots -- Abdallah Mohamed Rajab al Masri, also known as Abu Khayer, Abdel Aziz al Masri, and Abu Mohamed al Masri -- are in Iran. Authorities believe them to be, respectively, the head of al-Qaida's shura, or leadership council; a biological weapons expert who heads the terror network's effort to develop weapons of mass destruction; and its top explosives expert and training camp chief.