Best Evidence of Passengers:
Manifests - These are legal documents prepared by the airlines before departure based on check-in information. The manifest for Flight 11 was first made public on September 13th, 2001 in the Boston Globe. The rest did not become public until the Moussaoui trial in 2006.
Der Spiegel interviews - witnesses told magazine a week after 9/11 that they saw passengers at Gate 26 of Logan Airport around 7:30 AM (some sources say Flight 11 took off from this gate minutes later).
Newspaper accounts of passengers boarding Flight 93 - reports indicate passengers were delayed getting to plane by construction.
Source: Planes without Passengers: the Faked Hijackings of 9/11
Can you think of other evidence in favor of passengers boarding any planes in connection with the 9/11 plot?
Is this evidence sufficient to form PROOF of the existence of passengers?
To construct a falsifiable statement about the passengers, what conceivably would falsify the idea that there were no passengers?
Book cover for Planes and Passengers by Dean Hartwell