By developing a 'no blame' safety culture. Both the aviation industry and nuclear industry are pretty good examples. Oil & gas is getting there. Everybody is entitled to raise safety concerns without fear of retribution or assumptions being made. Difficult to achieve, but that's the aim.

I don't think you can ever have a 'no blame culture' but I do believe a 'low blame culture' or 'just culture' if you prefer the Reason terminology is desirable and achievable. If the prevailing culture has a tendency to seek blame and retribution as opposed to finding out the underlying causal factors, then this will inhibit the investigation process and ultimately prevent learning from incidents. For a proper investigation there should be some basic principles which are applied. This should then prevent senior management from jumping the gun, identifying potential suspects and sometimes . The first principle is to allow the lead investigator an unfettered access to evidence including witness statements. The second principle is that no one except the investigator and his team should be allowed to review the witness statements regardless of the seniority of the managers. Witness statements must be treated as confidential documents. A thorough investigation takes a great deal of time and effort - patience is a virtue. I have on occasions been asked to provide a summary of the investigation which I am reluctant to do whilst the investigation is on going. If pressed I provide no more than the basic facts. People can speculate if the choose but ultimately it is for the full investigation report to determine the immediate and underlying factors with recommendations and subsequent actions.

IMO, accident investigations should be in the H&S policy clearly stating it's propose and procedure, and conclusions not to be drawn until the investigation is complete. This should be stated clearly at the beginning of any interviews, but going back to your original point, the managers who have asked you to do the investigation.