Marriage records, parents' obituaries, voter rolls, property records and professional licences can trace people whose surnames changed over decades.
They are often inaccurate, so confirm any address through at least one independent public record before attempting contact.
A short, respectful letter explaining who you are, how you found them and that they may decline usually works best.
Respect the refusal, record it and stop contact; tell the investigating agency if you believe they hold important information.
No. Keep witnesses' contact details private and secure, and never publish current addresses or phone numbers.