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Microsoft said it suspects a former employee of an antivirus software firm was behind the Kelihos botnet attacks.
Russian citizen Andrey Sabelnikov "wrote and/or participated in creating" the harmful software which infected thousands of machines, Microsoft said.
Kelihos was used for sending out spam and spreading malware until it was "neutralised" in September 2011.
In a blog posting, the Microsoft's lead attorney warned that thousands of PCs remain infected with Kelihos' software.