The group allegedly behind the hacking attack on an IT company in the USA is demanding a ransom of 70 million dollars.
Then data should be released again, the group called "REvil" posted on Monday on their blog.
An expert from cybersecurity firm Recorded Future said the entry appeared to be real.
The group has been running the blog since last year.
In one of the largest blackmailing hacker attacks, thousands of companies around the world may have been paralyzed since Friday afternoon.
The hacker group "REvil" is suspected of having hijacked the desktop management tool VSA from Kaseya and installed a malicious update that infects thousands of customers of the US tech management provider.
Entire accounting systems were blocked by the hackers' encryption.
The attack has effects as far as Europe.
According to the Federal Office for Information Security (BSI), thousands of computers in Germany are also affected.
Most recently, there were several cyber attacks on US companies. In May, the colonial oil pipeline in the USA and the US subsidiary of the world's largest meat producer JBS fell victim to cyberattacks with ransomware. Last year, hackers gained access to the systems of ministries, authorities and companies using software from the American IT company SolarWinds.