My dad underwent ONE FBI investigation to get a clearance, and they DID go back and question people he went to high-school with.
A fraternity brother in college applied for an internship with the FBI. Two agents showed up at my apartment door, unexpectedly, asking for me by name and if they could visit with me about “Matt”.
There was an awkward moment as I inspected their credentials. It might have gone smoother had Matt informed me this might be coming. I was completely honest with them.
“Do you know if Matt has ever smoked pot?” -—> “YES. Matt has smoked pot.”
He did get the internship. Later, Matt told me that he thinks he got the internship because his references were so honest and consistent when they did the background check. I thought I had doomed him.
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.
In a previous life, I was a manager at a facilty which paid even crappier wages to their employees than I what was paid. The FBI showed up to question me one day about a kid who had worked there a couple of summers before. They showed the badge and the whole 9 yards. I thought he might be in some kind of trouble. Turned out that he had just applied to the academy and they were checking the application. He told me later he’d gotten in.