U.S. authorities have placed two Russian men on their most-wanted list of cybercriminals, bringing the total number of Russian suspects on the list to four.
The FBI is seeking the arrest of Andrei Taame, 33, and Alexei Belan, 26, in connection with separate crimes involving computer-related fraud, the agency said on its website this week.
The men are suspected of having been involved in a massive malware scam that targeted Apple's iTunes store and space agency NASA.
The FBI has offered a $100,000 reward for information leading to Belan's arrest and $50,000 for information resulting in Taame's arrest.
Russia's constitution forbids it from extraditing its citizens to other countries, and Moscow does not have an extradition treaty with Washington.
Taame, born in Syria, is wanted for his suspected involvement in a scam to infect more than 4 million computers in more than 100 countries with malware that connected Internet users to fraudulent servers controlled by his Estonia-based conspirators.
Taame and his accused conspirators reaped more than $14 million with the set-up, in part by re-routing users trying to load websites such as Apple's iTunes store, the movie rental service Netflix and the portal of the U.S. Internal Revenue Service to sites owned by unrelated businesses, according to a 2011 indictment in a U.S. federal court.
The hackers are accused of collecting fees from the owners of the websites to which the malware directed traffic in the scheme, known as "clickjacking."
The malware also engineered replacement advertisements that users of infected computers saw on websites including online retailing giant Amazon.com, sports news behemoth ESPN.com, and the website of the daily The Wall Street Journal, depriving the targeted businesses of actual advertising revenue, according to the indictment.
More than 10 NASA computers were infected with the malware, causing more than $60,000 in damages, according to the indictment.
Taame has been charged with conspiracy to commit wire fraud, conspiracy to commit computer-related fraud, wire fraud and computer intrusion, the FBI said, adding that he may have traveled to Cyprus or Russia.
Two of Taame's alleged conspirators have been extradited to the United States from Estonia. One of the men, Valeri Alexeyev, pleaded guilty in February to charges of conspiracy to commit wire fraud and conspiracy to commit computer intrusion.
The U.S. District Court for the Southern District of New York last week sentenced Alexeyev to eight years in prison and ordered a forfeiture of $7 million against him.
Belan, meanwhile, is accused of hacking into the networks of three major U.S.-based e-commerce firms in Nevada and California and reselling their user databases after encrypting the passwords of millions of the hijacked accounts, according to the FBI.
U.S. authorities have issued two separate federal arrest warrants for Belan, who was born in Latvia, the FBI said.
He has been charged with using a computer to obtain information from a protected computer, possession of 15 or more unauthorized access devices, aggravated identity theft and fraud in connection with a computer.
The FBI said that Belan was last known to be in Athens, Greece, and that he "may travel" to countries including Russia, Greece, Latvia, the Maldives and Thailand.