The United States has announced charges against four men including two Russian spies, accusing them of a massive data hacking of Yahoo in 2014 that affected at least half a billion accounts.

The two allegedly Russian spies and members of Russia's state security organization FSB are identified as Dmitry Dokuchaev and Igor Sushch.

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The massive hacking allegedly started when the two Russian FSB agents paid two cyber mercenaries to exploit the breached Yahoo data. The two alleged cyber mercenaries arrested are a 22-year old dual Canadian-Kazakh national Karim Baratov, and 29-year-old Alexsey Alexseyevich Belan, a Russian national living in the U.S.

According to CNBC,  acting assistant attorney general Mary McCord was quoted as saying that Belan is a "notorious" criminal hacker - one of the FBI's most wanted - known for hacking U.S. e-commerce companies.

Belan used the Yahoo attacks to launch spam campaigns, searched user communications for credit card and gift card numbers, and other schemes to "line his own pockets with money."

Fortune reports that the 39-page indictment laid by the Justice Department described several criminal activities ranging from spying to spamming, from scheming to market scheming to market drugs to using stolen Yahoo accounts to target Gmail accounts of particular individuals.

Targeted Gmail accounts are exploited as they are usually used as an alternative contact to Yahoo. As the indictment explained, the Russian FSB agents were paying the hackers $100 for each successful attempt to trick the owners of Gmail accounts to give up their passwords.

The U.S. government claims that the Russian targeted more than 50 Gmail accounts, but it remains uncertain how many of these were successful.

The indictment cited for example how one of the arrested hackers searched for three digital security codes of credit cards required for online purchases from compromised accounts, while others targeted Apple and iTunes accounts, and gift cards.