A South Korean cyber security expert says there is more circumstantial evidence that North Korea may be behind the global "ransomware" attack.
He said the way hackers took computers hostage was similar to previous cyber attacks attributed to North Korea.
Simon Choi, a director at anti-virus software company Hauri, said that North Korea is no newcomer in the world of Bitcoin and it has been mining Bitcoin using malicious computer programmes since 2013.
Last year Choi accidentally spoke to a hacker traced to a Pyongyang internet address about development of ransomware and he alerted South Korean authorities.
Researchers at Symantec and Kaspersky Lab also found similarities between WannaCry malware in the latest cyber attack and previous attacks blamed on North Korea.