Security researchers have tied the recent spate of digital breaches on Asian banks to North Korea, in what they say appears to be the first known case of a nation using digital attacks for financial gain.

In three recent attacks on banks, researchers working for the digital security firm Symantec said, the thieves deployed a rare piece of code that had been seen in only two previous cases: the hacking attack at Sony Pictures in December 2014 and attacks on banks and media companies in South Korea in 2013. Government officials in the United States and South Korea have blamed those attacks on North Korea, though they have not provided independent verification.

On Thursday, the Symantec researchers said they had uncovered evidence linking an attack at a bank in the Philippines last October with attacks on Tien Phong Bank in Vietnam in December and one in February on the central bank of Bangladesh that resulted in the theft of more than $81 million.

“If you believe North Korea was behind those attacks, then the bank attacks were also the work of North Korea,” said Eric Chien, a security researcher at Symantec, who found that identical code was used across all three attacks.