The first 9/11 conspiracy theories appeared on the internet just hours after the attacks, on 11 September 2001, and have grown in size and scope ever since.
Extensive reports by the 9/11 Commission, US government agencies and expert groups have debunked the existence of any hidden conspiracies.
But activist groups in the US and elsewhere, the 9/11 Truth movement, say the facts have been hidden.
Some leading members of the movement have also embraced conspiracies about Covid-19 and vaccines.
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