Hackers have again gained access to the world’s largest system for transferring funds among banks, a breach the network’s operator said indicates a wide-ranging effort to penetrate the financial system.

The Society for Worldwide Interbank Financial Telecommunication, a cooperative that runs the international messaging system between banks, said the attack targeted a commercial bank and managed to send Swift messages using the bank's valid codes. It followed the theft in February of $81 million from Bangladesh's account at the Federal Reserve Bank of New York.