The Last Straw by William J. Smith

“There’s absolutely nothing we can pin it down to with any real certainty,” Kessler said. “No mechanical defects that we’re sure of, no sabotage we can put our finger on, no murder or suicide schemes, nothing! We’ve put that plane back together so perfectly that it could almost fly again! We’ve got dossiers an inch thick on practically everybody who was aboard, crew and passengers. We’ve done six months’ work and we don’t have one single positive answer. The newspapers were yelling about the number of insurance policies issued for the flight but none of them looks really phony.”