Can't find this thriller

[ggrod]

I read this book back in the 90's, and it was thriller/horror about a killer who stalked and killed families. In this book the killer talks about other families he's murdered. But this particular family has created their own language they use around the house. This really intrigues the killer. I also believe the killer learned this family's language from stalking them. I think the killer was trained or hired by the government as well. Does anyone know what book I mean? Thanks!