The circumstantial evidence is compelling, but doesn't really prove anything.
True. But since Rosie Ruiz, new methods are supposed to catch people. According to wikipedia.com on the Aftermath of the Rosie Ruiz scandal:
As a result of the scandal, the Boston Marathon and several other races instituted a number of safeguards against cheating that are still used today. These include extensive video surveillance and the transponder timing RFID system that monitors electronically when runners arrive at various checkpoints on the course. These techniques have been used to identify other would-be cheaters, notably 'Jean's Marines', a group of charity runners who were caught cutting the course during the 2005 Marine Corps Marathon." http://en.wikipedia.org/wiki/Rosie_ruiz