The Swift network is warning its member banks hackers are refining their tools. Pictured is a padlock displayed at the Alert Logic booth during the 2016 Black Hat cybersecurity conference in Las Vegas, Aug. 3, 2016. David Becker/Reuters

A letter from the Swift network to member banks warns hackers are refining their cyberattack tools and the global bank transfer system remains vulnerable.

Reuters reported Monday the letter, dated Nov. 2, warned the threat “is very persistent, adaptive and sophisticated — and it is here to stay.”

Reuters said the letter is evidence the Belgium-based cooperative — Society for Worldwide Interbank Financial Telecommunication — remains vulnerable a year after $81 million was stolen from Bangladesh’s central bank.

Among the tactics now being used by hackers is software allowing technicians access to computers for technical support.

"We unfortunately continue to see cases in which some of our customers’ environments are being compromised," the letter said, noting a “meaningful” number of attacks have been made on both central and commercial banks, and 20 percent of them have resulted in stolen funds.

Bangladeshi officials told Reuters several central bank officials enabled the theft, which was the result of malware inserted into the bank’s system.

Bitcoin News Service reported the central bank has managed to recoup a portion of the stolen funds from a casino in the Philippines. Investigators determined the money was sold on the black market to a foreign exchange broker who transferred the funds to three casinos.

A Philippines court ordered the Solaire Resort and casino to surrender the money to Bangladesh. Some $10 million was handed over.

A commercial bank in Ecuador said it was held up for $12 million last year. In Vietnam thieves tried and failed to make off with $1.1 million in what investigators said may have been a practice run for the attack on Bangladesh.

Russia security services warned earlier this month that foreign spy agencies were preparing attacks on Russian banks in dozens of cities to destabilize “the financial system of the Russian Federation.” The Federal Security Bureau didn’t specify who was preparing the attack.