Edwards confirmed that Srizbi was still offline. "And we have confirmation that McColo traffic has not been re-hosted somewhere else," he added. "The backers of both are still scrambling." McColo was still unavailable as of mid-afternoon Tuesday.

Srizbi, which also goes by "Mailer Reactor," was among the world's biggest botnets. In April, noted botnet researcher of estimated Srizbi as composed of 315,000 infected PCs. The McColo takedown, Stewart said last week, had compromised computers -- aka "bots" -- from their criminal controllers.