By Tom Bergin and Jim Finkle LONDON/BOSTON (Reuters) – Cyber attacks targeting the global bank transfer system have succeeded in stealing funds since February’s heist of $81 million from the Bangladesh central bank as hackers have become more sophisticated in their tactics, according to a SWIFT official and a previously undisclosed letter the organization sent to banks worldwide. The messaging network in a Nov. 2 letter seen by Reuters warned banks of the escalating threat to their systems, according to the SWIFT letter. "The threat is very persistent, adaptive and sophisticated – and it is here to stay," SWIFT said in the November letter to client banks, seen by Reuters.