Flat Sale Fraud

I had purchased a flat in Mumbai in the year 2016 with all legal procedure including obtaining society NOC, Stamp duty payment, municipal registration, Society maintenance bill payment, electricity meter installed under my name. 10 months after my purchase I got a notice from the bank saying that the 1st owner had sold the flat to their customer in 2015 and as their customer did not pay the loan amount they will take possession of the flat. If the first owner had sold the flat to someone else how did he sell the same flat again to me? Now the bank has sealed the flat and I don’t understand how to catch the owner. The deal was done by the investor and the agent and the owner had come at the time of the registration. The investor and the agent are not answering my calls. The society secretary whom I met before the deal and who gave a clearance as well as share certificate is not available to meet me. My advocate had filed application for FIR against the first owner and the bank but it is still not lodged. Can you kindly advise what step I should take to bring the culprits to book?