One afternoon in the late 50s early 60s I was home alone and I guy with a badge (don't remember which, probably FBI) came to our door and asked to talk to my parents. Told him they weren't home.
He told me a (unnamed) neighbor was applying for a government job and he had some questions. He just asked if I had seen or heard of any unusual behavior. I said no, and he left and went to the next house. I don't remember if he came back later to talk to my parents. Scared the crap out of me.
In general, the FBI does the field interviews for security clearance investigations.
There's a separate agency to do clearance investigations. The name has changed several times, and just happened again in the fallout from a massive data breach. But, they don't have the network of field offices needed to do neighborhood and local law enforcement agency checks.
It may well have been a special agent for military intelligence (army)97B40. (MI agent/ counterintelligence agent) as during that time frame we, army intel, were doing all the security clearances in USA.