According to Reuters, the FBI investigation was based on an announcement from a security company Hold Security that it believed the Russian gang, called CyberVor, stole 1.2 billion credentials and as many as half a billion email addresses.
The FBI found posts in Russian hacking forums that a “mr.grey” either operated or had access to stolen data.
This “mister grey” had offered to make available account details for Facebook and Twitter users.
Hold Security revealed earlier this year that 420,000 websites were hacked, but this investigation is different.
US prosecutors have charged three men for alleged cyber crime offences for hacking into a number of websites, including JP Morgan Chase.