[If there were no passengers on the 4 9/11 flights...]
Hypothetically, it is not unreasonable to ASSUME that these people were actually involved in the events of that day and they were either silenced, or , my pet THEORY, taken to an underground city/lab. ( Too many valuable credentials combined to be coincidence )
Do some research on Denver international airport and you will get my drift.
( the most popular of many )
BTW, are you aware of the report of flight 93 landing in Cleveland that morning ?