Peter Bergen is CNN's National Security Analyst and the author of " The Osama bin Laden I Know ," which this article draws upon in part.

(CNN) Omar Abdel-Rahman, the Egyptian cleric who inspired terrorist plots in New York during the early 1990s and who died in an American prison on Saturday, was also the spiritual guide of key 9/11 plotters.

More specifically, he was the source of a laminated card of Arabic script that is critical to understanding why nearly 3,000 Americans lost their lives on the morning of September 11, 2001.

The Arabic on the card reads: "A fatwa [religious ruling] of the captive Sheikh Omar Abdel Rahman ... To all Muslims everywhere: Destroy their countries. [The Americans, Jews and Christians]. Tear them to pieces. Destroy their economies, burn their corporations, destroy their businesses, sink their ships and bring down their airplanes. Kill them in the sea, on land and in the air."

The author, who was jailed for life in 1996 for his role in terrorist conspiracies in New York, signed the fatwa: "Your brother Abdel Rahman, from inside American prisons." Explaining that his instructions were his final will and testament, he ordered his followers to: "Take my revenge on [the Americans] and do not let my blood be wasted in vain."

The fatwa was first publicly distributed by the leadership of al Qaeda at an extraordinary press conference in Afghanistan in May 1998. But its significance to the terrorist organization has largely gone unremarked.

Sheikh Rahman's fatwa was the first time that anyone associated with al Qaeda had given religious sanction to attacks on American aviation, shipping and economic targets. The fatwa, with its exhortations to "bring down their airplanes," "burn their corporations" and "sink their ships," would turn out to be a slowly ticking time bomb that would explode first on October 12, 2000, when a suicide attack blew a hole the size of a small house in the USS Cole in Yemen, and then again with even greater ferocity on 9/11.

The cleric's spiritual authority

To understand the significance of the fatwa, you have to understand the spiritual authority that its author, the militant cleric, exercised over al Qaeda. That terror group was led by Osama bin Laden and his deputy Ayman al Zawahiri, but neither of them had any standing as religious scholars.

Sheikh Rahman had a doctorate in Islamic jurisprudence from al-Azhar University in Cairo, the Harvard of Islamic thought. He also had a long history of guiding terrorist groups. Rahman had long been the spiritual guide of Egypt's two most violent terrorist organizations, members of which later occupied senior leadership positions within al Qaeda.

The special reverence that al Qaeda had for Sheikh Rahman was underlined by a two-hour propaganda videotape that the group's media division released in the spring of 2001, when the 9/11 attacks were in their final planning phase. Half way though the tape, in a segment entitled "Reasons," bin Laden explained why Muslims should wage a holy war against the United States.

Over a picture of Sheikh Rahman, bin Laden fumed, "He is a hostage in an American jail. We hear he is sick, and the Americans are treating him badly." Rahman is the only religious figure mentioned in the course of the two-hour videotape.

Indeed, the American incarceration of Sheikh Rahman was a hot-button issue for al Qaeda for many years. In 1997, during his first television interview, bin Laden told CNN that "Sheikh Omar Abdel Rahman is a Muslim scholar well-known all over the Muslim world. He represents the kind of injustice that is adopted by the US. A baseless case was fabricated against him even though he is a blind old man ... The US sentenced him to hundreds of years ... He is now very badly treated."

In September 2000, Al Jazeera aired a videotape of al Qaeda's leaders in Afghanistan sitting under a banner reading, "Convention to Support Honorable Omar Abdel Rahman." On the tape bin Laden vowed : "We promise to work with all our power to free our brother, Sheikh Omar Abdel Rahman." Off camera one of the sheikh's sons shouted, "Forward with blood."

Decades of sanctioning terrorism

The intense interest that al Qaeda took in the fate of Sheikh Rahman may come as something of a surprise to most Americans. If they remember him at all, Americans may dimly recall television images of the corpulent cleric when he lived in New York in the early 1990s, invariably dressed in flowing robes and a red felt hat denoting his senior clerical status, along with dark sunglasses that disguised his blind, opaque eyes.

On television, Sheikh Rahman came off like a cuddly Middle Eastern Father Christmas who had somehow morphed with Ray Charles.

The reality was more sinister: The blind sheikh was the Zelig of Islamist terrorism, repeatedly cropping up as the spiritual inspiration or instigator of the most spectacular terrorist attacks of the past two decades, from the 1981 assassination of Egyptian President Anwar Sadat in Cairo to the 9/11 plot itself.

Sheikh Rahman was important to al Qaeda not only because of the high regard the terrorist organization had for him as a religious scholar but also because he was long the spiritual guide of the Egyptian militants who were at the heart of al Qaeda's operation.

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The Egyptian terrorist organization, the Islamic Group, first came to prominence in Egypt during the 1970s when it started a campaign of robbing and killing Christian Copts. It was Sheikh Rahman who issued the fatwa that sanctioned the killings of Christians, according to Gilles Kepel , one of the world's leading authorities on Egypt's militant groups.

In 1973, Ayman al-Zawahiri, then a medical student, helped to found a sister organization to the Islamic Group. This was the Jihad Group, which confined its attacks to government officials and buildings.

Sadat's assassination

In early 1981, the sheikh agreed to act as the spiritual guide for both the Islamic Group and the Jihad Group, who joined forces in an effort to assassinate Egypt's President Anwar Sadat following a peace deal he had struck with Israel.

According to multiple news accounts, Sheikh Rahman gave his blessing to Sadat's assassination, and, on October 6, 1981, an army lieutenant named Khalid Islambouli sprayed Sadat with machine-gun bullets. Sheikh Rahman was subsequently arrested and charged with giving his imprimatur to Sadat's killing. Zawahiri was also charged, one of some 300 militants who were tried in the wake of Sadat's assassination.

In Zawahiri's 2001 autobiography, "Knights Under the Banner of the Prophet," he remembered Sheikh Rahman's "famous testimony" during the Sadat assassination trial with admiration : "Sheikh Rahman was roaring in the courtroom and speaking these words to the judge: 'I am a Muslim who lives only for his religion and is prepared to die for it. I can never remain silent while Islam is being fought on all fronts.'"

Rahman was acquitted on charges that he had ordered Sadat's assassination, even though he was found to have preached that "apostate" leaders should be overthrown.

The journalist Mary Anne Weaver interviewed Sheikh Rahman in 1993 about Sadat's assassination, eliciting a candid response about his role in the plot. Weaver asked , "At your trial in the Sadat assassination case you told the judge it was lawful to shed the blood of a ruler who does not rule according to God's ordinances."

The sheikh replied, "Yes, I told the judge that whoever does not rule as God orders is an infidel. And if you apply that rule to ... Sadat and Mubarak [Sadat's successor], they are all infidels."

By branding Sadat an "infidel," Sheikh Rahman was also branding him an apostate who had rejected his religion. To be an apostate is a major crime in Islam, and the sheikh's supporters would have immediately understood that anyone guilty of such a charge had to be killed. The blind sheikh's followers also tried to kill his successor, Hosni Mubarak. Indeed, Sheikh Rahman's involvement in a plot to kill Mubarak while he was visiting New York in 1993 is one of the charges that landed him in an American prison for life.

Advising from prison

Meanwhile, most of the conspirators in the 1993 World Trade Center bombing worshiped at mosques where Sheikh Rahman preached. The mastermind of the 1993 attack, Ramzi Yousef, in the only interview he ever gave, told the Arabic newspaper, Al-Hayat that he knew and admired Sheikh Rahman and that one of his goals was to "aid members of Egypt's Islamic Group and Jihad Group," the terrorist organizations that looked to the sheikh as their spiritual guide.

Following the 1993 attack, the sheikh's followers also planned a series of spectacular terrorist attacks in Manhattan against the United Nations building, the FBI's New York office and the Holland and Lincoln Tunnels. Those plots were averted because the FBI had inserted an informant into the sheikh's circle.

The informer asked the sheikh for his blessing for the attack on the UN building, but the sheikh waved him away from the operation on the basis that it would be bad for Muslims -- suggesting instead that the informant find a plan "to inflict damage on the army, the American Army."

That advice would lead to Sheikh Rahman's conviction in 1995 as part of "an organization whose aim was to wage jihad, or holy war, of terror against the United States" in the words of Mary Jo White, the prosecutor in the case.

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Despite his incarceration in an American jail, the sheikh's religious directives continued to spark violence in his native Egypt during the mid-1990s. On October 14, 1994, Naguib Mahfouz, the writer whose novels of Cairene life had won him the Nobel Prize in 1988, was stabbed and gravely wounded in an attack outside his house. The frail 83-year-old writer was lucky to survive the assault. Mahfouz's assailant later said he was carrying out a directive from Rahman.

In the United States, Rahman was largely forgotten once he began to serve his life sentence in 1996. After all, what damage could a blind, aging cleric do from his cell in a federal penitentiary?

Quite a lot, it turns out. A demonstration of the sheikh's enduring power came in 1997 when Egyptian terrorists massacred 58 tourists in Luxor, the site of several well-known monuments. The torso of one of the victims was split open by terrorists who inserted a leaflet calling for the release of Sheikh Rahman.

Facing a wave of popular revulsion against such tactics, the leaders of the terrorist Islamic Group negotiated a ceasefire with the government. Sheikh Rahman sanctioned the ceasefire agreement from his prison cell.

A year later, Rahman rescinded his support for the ceasefire because of the continued detention and torture of militants in Egypt. His withdrawal of support for the ceasefire in Egypt would resonate in Afghanistan with al Qaeda's leaders. In his 2001 autobiography, al Zawahiri explained "people of the stature of Omar Abdel Rahman ... oppose the [ceasefire] initiative."

The first and last press conference

The most important message the sheikh smuggled out of his cell was the fatwa calling on his followers to avenge his American imprisonment and "crash their airplanes," "burn their corporations" and "sink their ships." It is unclear when, or how, Sheikh Rahman was able to arrange to spirit this fatwa out of prison, but the fatwa made its first public appearance at the aforementioned press conference held by bin Laden at one of his bases in eastern Afghanistan in 1998.

The event, attended by a dozen or so Pakistani journalists, was noteworthy for two reasons: It was the first and last press conference ever given by al Qaeda's leaders. And it was also the moment when two of Sheikh Rahman's sons, Mohammed and Ahmed, would reveal themselves to be important players in al Qaeda.

Ismail Khan was one of the Pakistani journalists who attended the press conference. Khan recalls that bin Laden said that there was going to some sort of action by his group in the near future: "He spoke of some 'good news' in the weeks ahead." Nine weeks later, two US embassies in Africa were bombed within ten minutes of each other, killing more than 200 people.

During the course of the press conference, Sheikh Rahman's sons distributed laminated cards to the assembled journalists with their father's fatwa, calling for attacks on American aviation, shipping and corporations. (I obtained my copy of the fatwa from someone attending the conference.)

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Sheikh Rahman's sons introduced themselves to the journalist Ismail Khan by telling him that they planned to follow in the footsteps of their father and "continue the jihad." One son told Khan, "the US prison authorities are not treating father well ... They are killing him slowly."

Hamid Mir is a Pakistani journalist who was asked by bin Laden to write his authorized biography. As a result of that commission, Mir met with bin Laden and Zawahiri several times between 1997 and 2001. Mir says that Sheikh Rahman's fatwa had an important effect on Zawahiri, even more than on bin Laden. Mir told me, "The language by Sheikh Omar Abdel Rahman in his will is very, very strong. [Up until 1998] Bin Laden ... didn't issue any fatwa or anything against the ordinary Americans. He's against American troops, American government, but Dr. Zawahiri, he is against every American, because he is directly affected. His leader who is blind is arrested by the American authorities."

For al Zawahiri, and indeed all the Egyptians in leadership positions in al Qaeda, Rahman's fatwa to avenge his imprisonment by attacking American airplanes and corporations had the force of a religious order.

The fatwa's ripple effects

Sometime in mid-1998, Sheikh Rahman's fatwa to attack American targets also began to circulate in one of al Qaeda's key training camps in Afghanistan. At the Khaldan training camp, which has graduated several of al Qaeda's more dangerous alumni, Algerian Ahmad Ressam was learning how to make explosives, a skill he would later put into effect in an ill-fated attempt to bomb Los Angeles International Airport in December 1999.

At a subsequent terrorism trial, Ressam testified that while he was at the Afghan training camp he saw "a fatwa issued by Sheikh Omar Abdel Rahman with his picture on it, a piece of paper with his photograph on it." At the trial , Ressam was asked what his understanding of a fatwa was, and he replied: "A fatwa is something that a learned person would come up with. If there is an issue that people want an opinion on, the religious, learned man would study the issue and pass judgment on whether it was permissible or not."

Ressam, like other al Qaeda foot soldiers, understood that a fatwa from the learned Sheikh Rahman gave them a blanket religious imprimatur to attack and kill Americans.

Indeed, Sheikh Rahman's fatwa to attack the US economy and American aviation was one of the most important factors in the 9/11 attacks. Al Qaeda's Egyptian leaders wanted to exact revenge on the United States for the imprisonment and "ill treatment" of their spiritual guide. At the same time, Sheikh Rahman, as he had so often in the past, gave his followers his spiritual sanction for terrorist attacks.

Sheikh Rahman's fatwas are the nearest equivalent that al Qaeda has to an ex cathedra statement by the Pope. As someone with a doctorate in Islamic law, Sheikh Rahman was able to rule that it was legally permissible, and even desirable, to carry out attacks against American planes and corporations -- exactly the type of attacks that took place on 9/11.

Rahman's death in an American prison on Saturday will almost certainly spark calls from al Qaeda's current leader, al Zawahiri, for further anti-American attacks.