A year after that operation was called off, according to US and Saudi officials, from his Iranian refuge Saif al-Adel authorized al-Qaeda's branch in Saudi Arabia to launch a series of terrorist attacks in the Saudi kingdom that began in the capital Riyadh in May 2003, a campaign that killed scores of Saudis and expatriates.

On the face of it, the fact that a number of al Qaeda leaders and operatives and bin Laden family members found shelter in Iran is puzzling, as the Shia theocrats in the Iranian regime are hostile to the Sunni ultra zealots in al Qaeda, and vice versa.

For al Qaeda's operatives, life in Iran was more secure than for many of their colleagues in Pakistan who risked capture by Pakistani forces working with the CIA or death by CIA drones.

The Iranian regime likely saw the al Qaeda operatives as useful bargaining chips with the United States in the event of some kind of peace negotiations with the Americans. That peace deal, of course, never happened.

Of course, for Iran the adage, "the enemy of my enemy is my friend" may have also come into play, although there doesn't seem to be evidence that Iran and al Qaeda have ever cooperated on a specific operation.

That said, the 9/11 Commission found that of the 19 hijackers, "8 to 10 of the 14 Saudi "muscle" operatives traveled into or out of Iran between October 2000 and February 2001." Whether this was with any degree of Iranian complicity is still an open question.

The fact that leading members of al Qaeda were based in Iran from 2002 on was known to the U.S. government at the time. (In fact, in early 2003 counterterrorism officials briefed me about this development).

There is something of an irony here. This was during the same time period in which senior administration officials under President George W. Bush were citing the alleged presence of al Qaeda members in Baghdad and a supposedly burgeoning alliance between al Qaeda and Iraq's leader Saddam Hussein as a key reason to go to war against Saddam, Iran's bitter enemy.

Five years after the invasion of Iraq, the Senate Select Committee on Intelligence concluded, as had every other official investigation of the matter, that in fact there was no "cooperative relationship" between Saddam and al Qaeda.

In late 2008 al Qaeda operatives kidnapped Heshmatollah Attarzadeh-Niyaki, an Iranian diplomat, in the western Pakistan city of Peshawar.