The notorious hackers behind a string of banking heists have left behind a clue that supports a long-suspected link to North Korea, according to security researchers.

The so-called Lazarus Group has been eyed as a possible culprit behind the heists, which included the February 2016, $81 million theft from Bangladesh’s central bank through the SWIFT transaction software.

However, hackers working for the group recently made a mistake: They failed to wipe the logs from a server the group had hacked in Europe, security firm Kaspersky Lab said on Monday.

The logs show that the server had at one point connected with an Internet Protocol address from North Korea, an unusual sign given that the country has very limited internet access.

The IP address adds to the evidence that the Lazarus Group is tied to North Korea. Last year, security researchers also noticed similarities between coding techniques the group used in its banking heists and those used in the 2014 Sony hack, which the U.S. has blamed on North Korea.

That prompted speculation that North Korea was using the bank thefts to build up its foreign currency reserves.

In a blog post, Kaspersky Lab said the recovered server logs show a direct link between North Korea and a subunit within Lazarus that is devoted to hacking for profit. In addition to banks, the group has been targeting casinos, companies that develop financial software, and digital currency businesses.

The hackers probably failed to wipe the server because they had installed software to generate Monero, a cryptocurrency. “The software so intensely consumed system resources that the system became unresponsive and froze,” the security firm said.

However, Kaspersky Lab stopped short of naming the North Korean government as responsible for Lazarus's activities.

"As researchers, we prefer to provide facts rather than speculations," it said.

Lazarus has been around at least since 2009, and it appears to be a large organization, according to Kaspersky Lab. For example, it’s been found pumping out new malware while avoiding the use of old tools. Doing that requires a large-scale operation.

The hacking collective's newest malware strains have been found infecting victims in at least 18 countries. It has spread the malware using watering hole attacks, which involve hacking websites that specific targets might visit.

“All this level of sophistication is something that is not generally found in the cybercriminal world,” the security firm said. It also means that the group needs a lot of capital to keep functioning.

U.S. investigators are also building a case that would accuse North Korea in the $81 million bank heist in Bangladesh, according to a Wall Street Journal report on Monday.