The FBI has claimed that the second phase of 'Operation Bot Roast' has uncovered $20m in illegal activity and led to eight arrests.

Seven FBI offices participated in the second phase of the anti-botnet campaign, which resulted in 13 search warrants leading to the eight arrests.

The most high profile arrest was that of Ryan Brett Goldstein, a 21 year-old accused of launching a denial of service attack on the University of Pennsylvania.

Goldstein is believed to be an accomplice of the New Zealand teenager accused of masterminding a £12m global botnet operation.

The FBI said that the operation was a joint effort with the US Secret Service and the New Zealand Police.

In separate cases, the FBI arrested suspects in Washington, Kentucky and California on charges of operating botnets used for spamming and denial of service attacks.

Another arrest was that of John Schiefer, a 26 year-old Los Angeles resident described by the FBI as a "well-known member of the botnet underground".

The bureau alleges that Schiefer dealt in stolen data, including user names and passwords.

The FBI also arrested two Florida men accused of operating a multimillion dollar phishing operation. The two set up a bogus corporation to funnel money from other businesses.

FBI director Robert Mueller said that the arrests underscore the growing dangers posed by botnets.

"Bot Roast II [has revealed] the diverse and complex nature of crimes being committed through the use of botnets," he said.

Security experts have issued similar warnings. McAfee predicted earlier this month that botnet herders will employ more sophisticated methods of expanding and operating botnets over the next year.