This is standard practice for someone applying for a security clearance. Not you, but you have been named as a reference, friend, or in some cases simply a neighbor of someone applying. I would call and find out more facts, which they will share with you over the phone before any undo worrying.
PS. When my neighbors were questioned about me and my clearance application they all came to tell me. They wondered what the heck I was getting into that the FBI would come to people's houses and ask their questions. But it is entirely for the benefit of the prospective employee. If they are asking about a neighbor, don't try to cover up for them. Its part of your job to be honest. If it is about you, then I agree that you should see a lawyer too.
BUT they said it had to do with a "criminal investigation". They also said, after I asked, that I was "not currently under criminal investigation".
I have talked to them three times on the phone. Twice briefly and once at some length. The agent would not budge as far as giving info went.
I am calling criminal defense lawyers this morning.
Thanks