The fount of all knowledge.(FOAK)
Almost every neighbourhood, block of flats, or social network, (even a golf club) which the victim and the convicted person operated in has someone who knows everybody, knows all the theories, all the backgrounds - and is keen not only to tell you, but to even make inquiries for you from her ( it's usually a woman) trusted position in society.
Finding this person is usually easy. Go from door to door saying that you are looking for a friend who lived in the area, but who may have moved ( always leave these things vague - and therefore pliable). You want to find someone who has lived in the area a long time,. who knows a lot of people and who may remember your friend. Be prepared in case you actually stumble over the FOAK; but otherwise, be assured that you will not go on doing this for long before someone sends you to her.
FOAKs are what they are for a reason - and it is often something you can help them with. Be very friendly - they need friendship. But also do their shopping, give them chocolates, if they have no family - but above all, let them talk - about anything and everything - until you have their confidence..