Two Russians among four charged after MI5 contributes to Yahoo hacking probe
MI5 made "substantial contributions" to an investigation which has seen four people - including two members of the Russian security services - charged over one of the largest data breaches in US history.
The US Justice Department announced on Wednesday that charges have been brought in relation to the 2014 attack which targeted around 500 million Yahoo accounts.
Acting Assistant Attorney General Mary McCord said: "We're here for one of the largest data breaches in US history.
"Today we are announcing the indictment of four individuals responsible for the 2014 hacking... of Yahoo, the theft of information about at least 500 million Yahoo accounts, and use of that information to obtain the contents of accounts at Yahoo and other providers."
FBI executive assistant director Paul Abbate said: "We are extremely grateful as well to our international partners for their assistance and support leading up to these criminal charges today.
"Those partners include Canada's Royal Canadian Mounted Police and, as mentioned, the Toronto police service and their fugitive squad.
"As well, the United Kingdom's MI5 made substantial contributions to the advancement of this investigation also."
He added that the indictment details how Russian federal security service officers worked together with criminal hackers and "conspired to plan and carry out one of the largest cyber intrusions in US history".
The accounts and personal details of around 500 million Yahoo users were compromised in the breach, and millions of users' contacts were also stolen.
Mr Abbate said the aim of the alleged theft was to enable the alleged perpetrators to carry out fraud schemes for their own personal financial gain and enrichment.
Two of the arrested men are said to be Russian Federal Security Service officers, who allegedly "directed, facilitated and paid" the hackers.
"They worked with co-conspirators to hack the computers of American companies providing email and internet-related services," Ms McCord said.
Although the Justice Department has previously charged Russian hackers with cyber crime - as well as hackers sponsored by the Chinese and Iranian governments - this is the first criminal case brought against Russian government officials.
Yahoo only disclosed the 2014 breach last September when it began notifying the affected users that information such as their email addresses, birth dates, answers to security questions and other personal details may have been stolen.
Three months later, it revealed it had uncovered a separate hack in 2013.
In that attack about one billion accounts, including some that were also hit in 2014, were targeted.