Researchers have found more digital fingerprints tying this month's WCry ransomware worm to the same prolific hacking group that attacked Sony Pictures in 2014 and the Bangladesh Central Bank last year.

Last week, a researcher at Google identified identical code found in a WCry sample from February and an early 2015 version of Contopee , a malicious backdoor used by the hacking team Lazarus Group. The group has been operating since at least 2011. Additional fingerprints linked Lazarus Group to hacks that wiped almost a terabyte's worth of data from Sony Pictures and siphoned a reported $81 million from the Bangladesh Central Bank last year . Researchers say Lazarus Group carries out hacks on behalf of North Korea.

On Monday, researchers from security firm Symantec presented additional evidence that further builds the case that WCry, which is also known as WannaCry, is closely linked to Lazarus Group. The evidence includes:

The discovery of three pieces of malware previously linked to Lazarus Group that were left on a network hit in the first-known infection of WCry in February. The malware included Trojan.Volgmer and two variants of Backdoor.Destover, the disk wiping tool used in the Sony Pictures attacks.

Trojan.Alphanc, which was used to spread WCry in attacks that took place in March and April, is a modified version of Backdoor.Duuzer, which has previously been linked to Lazarus.

Bravonc, another trojan used to install WCry onto computers in earlier attacks, used the same IP addresses for command and control as Duuzer and Destover.

Bravonc has similar code obfuscation as WCry and Infostealer.Fakepude, another piece of malware linked to Lazarus Group.

Newly discovered similarities between Contopee and the WCry ransomware itself.

The earlier versions of WannaCry and the one used in the May 12 attacks are largely the same, with some minor changes, chiefly the incorporation of the EternalBlue exploit. The passwords used to encrypt the Zip files embedded in the WannaCry dropper are similar across both versions ("wcry@123", "wcry@2016", and "WNcry@2ol7") indicating that the author of both versions is likely the same group. The small number of Bitcoin wallets used by first version of WannaCry, and its limited spread, indicates that this was not a tool that was shared across cyber crime groups. This provides further evidence that both versions of WannaCry were operated by a single group.

The similarities in tools, techniques, and infrastructure, Symantec researchers said, make it "highly likely that Lazarus was behind the spread of WannaCry." In a blog post , they wrote:

In addition to the previously discovered identical code found in both WCry and Contopee, Symantec researchers say one variant of Contopee uses a custom Secure Sockets Layer implementation—including a set of 75 different ciphers—found in WCry. The OpenSSL crypto library, by comparison, provides more than 300 ciphers, making it unlikely both pieces of malware would offer precisely the same subset.

Security researchers have long warned that attributing hacking operations and malware to specific groups is an imprecise undertaking that's frequently fraught with errors. Readers should keep those caveats front and center as they digest Symantec's findings. Still, the new similarities indicate that industry-wide agreement is growing that Lazarus Group was somehow involved in the WCry outbreak earlier this month. Don't be surprised if additional researchers unearth new similarities.