You haven't yet gotten a straight answer to your original questions, mainly because this is an old and very frustrating story. The short answer is: yes, it could have been the TSA people. Or it could have been the airline baggage handlers, helped by the fact that, as ralfp said, TSA don't check who (or what) goes in and out of the "secure" area where bags are loaded. What do you do? There's not much. If the loss is large enough, see if your travel insurance will cover it. You can write to the airlines and airports involved, but the TSA will say "It was probably the baggage handlers" and the airline will say "it was probably the TSA" and they will both stand there looking like guilty children saying "you can't prove it was us." It is very unlikely that either will admit anything.

Yeah, it's happened to a lot of us, and people we know. And as N965VJ points out, 128,000 other people.

If you check on this thread (link), there's a discussion about whether to lock your bag, and other ideas about having a photograph and inventory of the contents.

Some of the TSA people claim that it's the passengers fault if they put valuable items in checked baggage. To our TSA members, I have to ask: "Socks?????"