Federal prosecutors have filed charges against two Russian nationals accused of carrying out one of the largest cybercrime sprees in history, orchestrating a string of attacks on computer systems in the United States and targeting victims in at least 11 states.

The accused were responsible for tens of millions of dollars in damages, and the attacks netted them over $3 million, according to court documents.

Maksim Yakubets and Igor Turashev are accused of planting malware on computers, attacking several institutions in Pennsylvania — a bank, companies, a school district — in addition to targets in other states, including a lumber company, a natural gas company, and a small organization of nuns in Chicago, according to a complaint unsealed Thursday.

Yakubets, who is linked to Russia's Federal Security Service (FSB), was the leader of the hacking operation and Turashev was his assistant, prosecutors said. The two men, who were both indicted in Pittsburgh but whose whereabouts are unknown, obtained access to the U.S. computer systems through phishing emails claiming to be from legitimate companies and groups.

Igor Turashev, left, and Maksim Yakubets are accused of planting malware on victim computers. FBI

The attacks were among "the worst computer hacking and bank fraud schemes of the past decade," Assistant Attorney General Brian A. Benczkowski said at a news conference.

Benczkowski added that Yakubets was a "true 21st century criminal" and the "leader of a cybercriminal gang" who allegedly orchestrated "the kinds of criminal schemes so audacious and sophisticated they would be difficult to imagine if they were not real."

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The accused targeted, apparently unsuccessfully, the Sharon City School District in western Pennsylvania. But they had more success draining funds from the bank account of Penneco Oil.

The Department of Treasury has added the defendants to its Office of Foreign Assets Control list, which administers and enforces sanctions. The State Department and the FBI are offering a $5 million reward for information that leads to Yakubets' arrest and conviction.