They did not enter USA from Canada, and that has been debunked for a very long time. Here’s a quote from a 2009 article with a reference to the 9/11 commission report from 2004.
Can’t let facts stand in the way of xenophobia?
>>>That prompted Michael Wilson, Canada's ambassador to the United States, to set the record straight Tuesday.
Wilson told reporters at the Border Trade Alliance meeting in Washington, where he was keynote speaker, that he is "frustrated" that the 9/11 myth has surfaced yet again.
"Unfortunately, misconceptions arise on something as fundamental as where the 9/11 terrorists came from," he said.
"As the 9/11 commission reported in 2004, all of the 9/11 terrorists arrived in the United States from outside North America. They flew to major U.S. airports. They entered the U.S. with documents issued by the United States government, and no 9/11 terrorists came from Canada."
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