Incident investigation team varies in size depending on complexity and severity of the incident. At a minimum the team should include someone with incident investigation training. Include an experienced investigator to mentor that person if they are new to investigation. Beyond the investigation lead, need to include witnesses and at least one person who understands the process, operation area, machinery, etc. where the incident occurred. If you interview a Union represented worker, always include a Union representative. Other than the Union situation, it's best to interview workers without HR or supervisor present. You will get more open anf unbiased comments.

Regarding reporting results, i prefer to see a one page high level summary go to all employees early-on reporting the incident and a second one pager when the investigation is complete.

Yes! From the top down everyone should be included. Director should get a full report, Site Leader also, then Sr Managers, Supervisors and include pictures . These investigations should be lead by Director and Safety Manager. When these work hand in hand it promotes a safer workplace, the safety culture changes and becomes higher on the list.
We have won 3 out the last 5 gold Safety cups using this format this year 4 years ago they were at the bottom of the list. It works!