He may not actually know the person. (I don't really know you all) but he has had some contact with them. It would be impossible for me to find out who you were, but he would know that the authorities could/would do just that.
IOW he may be the key to solving the problem and that's why the DA is bringing him in, to help them unlock the door.
BTW I think the Ramsey's left their door unlocked or someone put a piece of tape on in to keep it from locking when they left. The perp came in the front door. JMO
I've always thought that they left a door unlocked in their hurry to go to the party.
Post Script.....Last chapter of his book....He's a big hero /sarc for helping the authorities solve the case....He "loved her".
Become a millionaire with the book royalties changes his name/sex to JohnBenet and live happily ever after.
Good day to y'all. Actually, with only a little bit of tech/web savvy, it wouldn't be hard at all, but your point is still well taken. Most internet users have no idea how un-anonymous they really are to determined snoops.