Cyber security researchers claim to have found technical clues they say could link North Korea with the global "ransom-ware" cyber attack.
"In each of these cases, the virus discovered was proven not to be the WannaCry virus, but an older virus for which protection was available".
With 49,000 computers being used by the HSE, everything is being done to ensure that patient care is unaffected.
A Microsoft security patch, released in March to close the vulnerability in Windows operating systems exploited by the ransomware virus, has been downloaded onto 28,000 HSE computers since Saturday.
A spokesperson for the Department of Health said it was not aware of any "similar incidents at this time" in healthcare organisations in Northern Ireland.
Precautions included removing the facility from incoming email list and removing the affected hardware from the system and placing patches on remaining equipment.
More than 200,000 institutions and organisations were infected by the "WannaCry" virus which struck in the United Kingdom and Spain first on Friday before spreading around the world. On Friday, Britain's NHS network was affected, shutting down many of its systems and affecting services.
"All external emails have been blocked since Friday evening as a precautionary measure while security work is undertaken, and staff now have no access to emails", she said.
Yesterday afternoon, the HSE also confirmed that nearly all the 1,200 servers that were rebooted to allow an anti-virus update to take effect were back online.
Staff members were unable to receive emails from internal and external sources, access the internet or external based servers.
Head of Europol Rob Wainwright said: "At the moment, we are in the face of an escalating threat".
A spokeswoman said that due to the threat its inward email system had been frozen, resulting in the organisation not receiving any notifications of child protection and welfare concerns via email since Friday.
Richard Corbridge, HSE chief operations officer, yesterday confirmed the attempts to attack computer systems across the health service had been as "virulent" as in other countries. "The numbers are going up, I am anxious about how the numbers will continue to grow when people go to work and turn (on) their machines on Monday morning", he said.
Emails into the HSE will continue to be blocked until Wednesday.