TWENTY years ago Al Qaeda was plotting the largest and most destructive terrorist attack in history.

       Here is what we know about the fanatic’s motivation to carry out mass murder.

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       Smoke pours from the twin towers of the World Trade Center after they were hit by two hijacked airliners in a terrorist attack Credit: Getty Why did Al Qaeda attack the United States?

       Before the 9/11 attacks, Al Qaeda issued proclamations.

       Some believe they provide insight into the motivations.

       One was the fatwā of August 1996 and the other was a shorter fatwa in February 1998.

       The terror group leader Osama bin Laden, penned a November 2002 "Letter to America".

       It read: “The expansion of Israel is one of the greatest crimes, and you are the leaders of its criminals.

       “And of course there is no need to explain and prove the degree of American support for Israel."

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       In the letter, the terror mastermind also said the creation of Israel was a crime that must be erased.

       He wrote: “Each and every person whose hands have become polluted in the contribution towards this crime must pay its price, and pay for it heavily.”

       Another motive was the presence of American troops in Saudi Arabia which was bin Laden’s homeland, following the 1991 Gulf war.

       Essentially he wanted to destroy American influence in all Muslim countries, destroy Israel and overthrow pro-Western governments in Muslim states.

       What happened on 9/11?

       On the fateful date, a group of al-Qaeda terrorists hijacked four passenger planes.

       American Airlines Flight 11 and United Airlines Flight 75 smashed into the North and South towers of the World Trade Center complex in New York City.

       8.46am - First American Airlines plane hits the World Trade Center’s North Tower

       9.03am - Twenty minutes later, a second jet crashed into the South Tower

       9.37am - A third plane crashes into the Pentagon in Washington DC

       10.03am - A fourth crashed in Pittsburgh, Pennsylvania as passengers fought with the hijackers

       9.59am - The South Tower was the first to collapse after burning for about 56 minutes.

       10.28am - The North Tower collapses

       Survivors of the Twin Towers terror attack tell their stories in Surviving 9/11

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