Two members of Russia's intelligence agency are among four people charged in a huge recent hack of Yahoo, U.S. law enforcement officials announced Wednesday.
Two criminal hackers also were part of the data breach that affected at least 500 million accounts and millions of user contacts, according to a federal indictment.
The agents, Russian FSB officers Dmitry Aleksandrovich Dokuchaev and Igor Anatolyevich Sushchin, are involved in work for the Russian Center for Information Security, which is the FBI's point of contact for cybercrime, said Mary McCord, acting assistant attorney general for the Justice Department's National Security Division.
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"There are no free passes for foreign, state-sponsored criminal behavior," McCord said.
NBC News has not received a response to the allegations from the Kremlin, spy agency FSB or Ministry of Foreign Affairs.
Though the Justice Department has previously charged Russian hackers with cybercrime — as well as hackers sponsored by the Chinese and Iranian governments — this is the first criminal case brought against Russian government officials.
"This is an unprecedented indictment," said John Bennett, FBI special agent in charge in San Francisco. "We take this seriously and the FSB should take this very seriously."
The announcement comes as federal authorities investigate Russian interference in the 2016 presidential election through hacking, which Russia has denied but is widely accepted in the U.S. intelligence community.
"This is explosive," retired U.S. Navy admiral James Stavridis, once NATO Supreme Allied Commander Europe, said on MSNBC, adding that the cyberattack "demands a response."
McCord, the Justice Department's top official on national security, said earlier Wednesday that Yahoo was hacked "with the backing of a nation-state," NBC News reported. She did not offer specifics at a seminar, sponsored by the Financial Times.
The FBI has had limited cooperation from the FSB's Center for Information Security in the past, including in requesting the return of one of the hackers on different charges in 2014, FBI Executive Assistant Director in Charge Paul Abbate said.
"We expect and hope for their cooperation here," Abbate added.
The other two named in the federal indictment are Alexsey Alexseyevich Belan, who has been indicted twice before and is on the FBI's "cyber most wanted" list, and Karim Baratov.
Baratov was arrested Tuesday in Canada, officials said.
The FSB agents allegedly paid Belan and Baratov to hack into emails in the U.S. and elsewhere. They broke into Yahoo in November or December 2014, taking user names, recovery email accounts, phone numbers and more.
The information had intelligence value to the FSB, while the hackers were able to use it to "line their own pockets," McCord said.
The indictment is a message to criminal hackers as well, Bennett said: "You can try and hide out in the corners of the dark web, but we will hunt you down."
Officials commended Yahoo and Google, whose customers were targeted in the breach, for being cooperative in the investigation.
"When you are going against the resources and backing of a nation-state, it is not a fair fight and not a fight you are likely to win alone," McCord said. "But you don't have to go it alone."
In a statement, Chris Madsen, Yahoo's assistant general counsel and head of global security, thanked law enforcement agencies for their work.
"We're committed to keeping our users and our platforms secure and will continue to engage with law enforcement to combat cybercrime," he said.