The conjecture here concerns the weapons used by the
hijackers. No evidence was found that any of the hijackers
on any of the four planes were armed with plastic knives,
and only on one plane, Flight 77, did a passenger report
that a hijacker had a weapon
resembling a box cutter.
The conjecture was elevated to fact by Attorney General
John Ashcroft when he told ABC News on September 15th
that "investigators believed that each of the commandeered
planes had been hijacked by groups of three to six men
armed with box cutters and plastic knives."