The vast majority of investigators are either professionally qualified pilots: most usually an ATPL, or Chartered Engineers. A few are both.
Cranfield do offer highly regarded specialist training in air accident investigation, but it is generally attended by people who are already highly experienced aviation professionals.
I may be misjudging, but I think that air accident investigation is much closer to academic research with the objective of creating recommendations about avoiding future accidents than road accident investigation, which I believe is much more about proportioning liability.