I agree with AmyLou100 and Notmoresugar it's not just the 2 people, it's their families too, so a lot of people have had their lives turned upside down by the death of their loved ones.
To me, this is a huge betrayal and shows he doesn't think too much of you...sorry to be so blunt as I'm sure you are hurting big time right now and I can't begin to think how I would feel in your situation but I do know I couldn't stay in the relationship. Not because of the accident (or whatever it was deemed to be)...but because of the deceit and lack of trust towards you.
Sure, he has a right to privacy, but he's your partner and you've been together 20 years, other people know the truth of what happened, but they have chosen to keep 'you' in the dark...why? This is not running over a neighbour's cat that he's kept quiet about, this is truly serious and the whole family (or at least those who know) must have put their heads together and planned not to spill the beans...they are making you the laughing stock.
Lancashirerose I truly hope you are OK and have someone outside this family to confide in? Can you stay with someone for however long it takes to get your head sorted? Sheesh, nobody could blame you for wanting that.
As for finding the details, if internet searches are not uncovering anything for you, maybe a private researcher could help...even someone into family history as they know lots of places to find historical information and it gives you breathing space while that is being done.
Please continue to post here so we know you are OK? Thinking of you 