The conspiracy theory that still won’t rest in peace: Think it’s only cranks who believe the Twin Towers attack was a U.S. government plot?

Just imagine Geoff Campbell’s last terrified moments, trapped on the 106th floor of the North Tower of New York’s World Trade Centre. Far below him, between the 93rd and 99th floors, an American Airlines Boeing 767 hijacked by Al-Qaeda terrorists and loaded with 20,000 gallons of aviation fuel had been flown into the building.

The South Tower had already collapsed in huge clouds of concrete dust after being hit by another passenger aircraft. As they were engulfed in lethal fumes, Geoff and scores of other men and women in the North Tower began smashing windows in a desperate attempt to gulp some fresh air, and begged for help in mobile phone calls to rescue teams and loved ones.

Some of those trapped with the young Briton on that morning of September 11, 2001, were later photographed hurling themselves to their deaths hundreds of feet below, rather than staying where they were to be burned alive or suffocated.

And then the North Tower collapsed, taking Geoff’s life and the lives of hundreds of others with it. We are now in the 20th anniversary year of 9/11, the worst terror atrocity the world has witnessed, the day when more than 2,600 people were killed in the Twin Towers and the surrounding area of downtown Manhattan.