A cyber security company has asserted that the hack of 500 million account credentials from Yahoo was the work of an Eastern European criminal gang.

The latest claim adds another layer of intrigue to a murky investigation into the unprecedented data heist

US-based InfoArmor issued a report whose conclusion challenged Yahoo’s position that a nation-state actor orchestrated the heist, disclosed last week by the internet company. 

InfoArmor provides companies with protection against employee identify theft. It said the hacked trove of user data was later sold to at least three clients, including one state-sponsored group. 

Reuters was unable to verify the report's findings, while Yahoo declined comment. 

The Federal Bureau of Investigation, which is investigating the hack, did not return a call seeking comment. 

A US government source familiar with the Yahoo investigation said there was no hard evidence yet on whether the hack was state-sponsored. 

Attribution for cyber attacks is widely considered difficult in both the intelligence and research communities. 

The task is made especially challenging by the fact that criminal hackers sometimes provide information to government intelligence agencies or offer their services for hire, making it hard to know who the ultimate mastermind of a hack might be.

Yahoo said last week that it only recently discovered the intrusion, which it blamed on a state-sponsored actor without providing technical evidence. 

Nation-state hackers are widely viewed as possessing more advanced capabilities than criminal groups, a perception that could benefit Yahoo as it works to minimize fallout from the breach and complete its sale to Verizon Communications.

InfoArmor concluded the Yahoo hackers were criminal after reviewing a small sample of compromised accounts, Andrew Komarov, the firm's chief intelligence officer, said in an interview.

The hackers, dubbed Group E, have a track record of selling stolen personal data on the dark web, and have been previously linked to breaches at LinkedIn, Tumblr and MySpace, Komarov said.