BA most certainly can help you. Whether it does so is a different question.

As this was an international arrival, the bag is with BA. Whether it is still in MIA or has been returned to LHR is an open question. It would surprise me if BA had already done so.

But, neither L&F nor AA have anything to do with it.

Just be patient as BA will need to clear it through Customs and Customs may (depending on circumstances) wish to examine it or speak with you. If CBP does, just be honest about it. People make mistakes.