Which nation-state is behind the sophisticated, stealthy Regin malware?

Which nation-state is behind the sophisticated, stealthy Regin malware?

‘Regin’ malware described as ‘groundbreaking and almost peerless’
“Its capabilities and the level of resources behind Regin indicate that
it is one of the main cyberespionage tools used by a nation state,”
Symantec said Sunday, explaining that “development took months, if not
years, to complete.”