My husband has gone through several throughout his career when working on T.S. DOD projects. They go back to high school for sure, and earlier if high school acquaintances were classmates before. They walk down your street and knock on doors. (The questions they ask your friends and neighbors can scare the crap out of them. Very detailed.) They go to your pastor(s), current and prior. They miss nothing on one investigation. They missed this on six? Puh-leeeze.
My dad told me they asked several of his high-school classes whether he had ever attended any Communist Party meetings in the 1950’s!