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While British prosecutors identified a former KGB agent living in Moscow as the prime suspect in the killing of former Russian spy Alexander Litvinenko, Russian authorities have focused on Mr. Berezovsky, Mr. Putin’s political foe.

On the same morning, the Web site of Ekho Moskvy, a liberal Moscow radio station where criticism of Kremlin policies can often be heard, was brought down by a DDoS attack.

Similar tactics have frequently been used by Western hackers — in 2000, the Web sites of CNN, Yahoo and EBay were paralyzed by online blackmailers. Massive attacks in 2002 and February 2007 attempted to disable the Internet itself.

The United States — especially the government sector — was the target of more than half of DDoS attacks worldwide, according to Symantec. The FBI recently arrested several DDoS hackers as part of “Operation Bot Roast” sting.

Nothing of the kind is happening in Russia.

Mr. Panfilov of the Center for Journalism in Extreme Situations said Russian opposition Web sites will find themselves under increasing pressure as election season heats up.

“There will be purges of online publications, shutdowns or takeovers of the last independent media outlets and strong pressure on Web users,” he said.

Associated Press correspondent Michael Baumann contributed to this report.