From the 9/11 Commission's report.
Assistance from Hezbollah and Iran to al Qaeda
As we mentioned in chapter 2, while in Sudan, senior managers in al Qaeda maintained contacts with Iran and the Iranian-supported worldwide terrorist organization Hezbollah, which is based mainly in southern Lebanon and Beirut. Al Qaeda members received advice and training from Hezbollah.
. . .
Our knowledge of the international travels of the al Qaeda operatives selected for the 9/11 operation remains fragmentary. But we now have evidence suggesting that 8 to 10 of the 14 Saudi "muscle" operatives traveled into or out of Iran between October 2000 and February 2001.121
In October 2000, a senior operative of Hezbollah visited Saudi Arabia to coordinate activities there. He also planned to assist individuals in Saudi Arabia in traveling to Iran during November. A top Hezbollah commander and Saudi Hezbollah contacts were involved.122
Also in October 2000, two future muscle hijackers, Mohand al Shehri and Hamza al Ghamdi, flew from Iran to Kuwait. In November, Ahmed al Ghamdi apparently flew to Beirut, traveling-perhaps by coincidence-on the same flight as a senior Hezbollah operative. Also in November, Salem al Hazmi apparently flew from Saudi Arabia to Beirut.123
In mid-November, we believe, three of the future muscle hijackers, Wail al Shehri, Waleed al Shehri, and Ahmed al Nami, all of whom had obtained their U.S. visas in late October, traveled in a group from Saudi Arabia to Beirut and then onward to Iran. An associate of a senior Hezbollah operative was on the same flight that took the future hijackers to Iran. Hezbollah officials in Beirut and Iran were expecting the arrival of a group during the same time period. The travel of this group was important enough to merit the attention of senior figures in Hezbollah.124
Later in November, two future muscle hijackers, Satam al Suqami and Majed Moqed, flew into Iran from Bahrain. In February 2001, Khalid al Mihdhar may have taken a flight from Syria to Iran, and then traveled further within Iran to a point near the Afghan border.125
KSM and Binalshibh have confirmed that several of the 9/11 hijackers (at least eight, according to Binalshibh) transited Iran on their way to or from Afghanistan, taking advantage of the Iranian practice of not stamping Saudi passports. They deny any other reason for the hijackers' travel to Iran. They also deny any relationship between the hijackers and Hezbollah.126
So who is this "senior Hezbollah operative" who was coincidentally travelling with numerous 9/11 hijackers in and out of Iran?
In an interview with FrontPage Magazine, Kenneth Timmerman says it's recently killed terrorist Imad Mugniyah.
I saw a report just yesterday suggesting that the Rev. Guards lured Mugniyeh into a trap into Damascus, to make sure that he never revealed the extent of his involvement with al Qaeda in the 9//11 plot. This was a secret the Iranians would have liked to ensure that Mugniyeh carried to his grave.
I’ve got news for them, though: the word is out. Just take a look at pages 240-241 of the 9/11 commission report, which describes in elusive terms the travel of eight to ten of the “muscle” hijackers in and out of Iran in the company of a “senior Hezbollah operative.” That operative was none other than Imad Mugniyeh.
FP: In other words, you are saying that Mugniyeh was involved with the 9/11 terror attack and so was Iran ˆ and the Iranians might have very well killed him so that the full extent of their own involvement would not become known.
Timmerman: There is absolutely no doubt that Mugniyeh and his masters in Iran were directly and materially involved in the 9/11 plot.