Jarvis, the rail maintenance contractor, has suggested sabotage was to blame. It says the nuts were found detached during a check on 1 May, and screwed back on by its engineers. A subsequent visual check on 9 May - the day before the crash - apparently found nothing wrong.
It has suggested the nuts may have been deliberately removed by someone with enough knowledge and "utterly perverse" intent.
However, neither the HSE nor British Transport Police have found any concrete evidence to support the sabotage theory.
Other experts and unions have suggested the missing nuts were down to maintenance errors.