Microsoft continued to put pressure on the operators behind the Rustock botnet, offering a $250,000 (around £155,000) bounty for information leading to the arrest and conviction of the person or group behind the spamming network.

The bounty will help advance the software giant's investigation, producing new leads to the authors' identities and perhaps giving law enforcement enough evidence to pursue a criminal case against the Rustock operators, says Richard Boscovich, senior attorney for Microsoft's Digital Crimes Unit.

Microsoft has already analyzed the evidence seized by U.S. Marshals and a third-party forensics firm in seven simultaneous court-sanctioned raids in mid-March. While the evidence led Microsoft to place advertisements in two Russian newspapers, summoning the operators to appear in U.S. court, the company has so far not identified the bot masters.

"The bounty in this case is really to provide us with more information to augment what we already have and to confirm whether or not we are on the right path," Boscovich says."It helps us narrow the scope, so you are not going down every rabbit hole -- reducing the amount of time tracking down bad leads."

The bounty marks the fifth time Microsoft has offered money from its $5 million Anti-Virus Reward Program to hunt down cybercriminals. When it announced the program in 2003, Microsoft offered two $250,000 bounties -- one for the author of MSBlast and one for the creator of the Sobig virus.

In May 2004, an 18-year-old computer science student, Sven Jaschan, admitted to creating the Sasser worm after authorities were tipped off by bounty-seeking acquaintances. In February 2009, Microsoft offered a bounty for the criminals behind the Conficker worm. So far, the bounties for MSBlast, Sobig and Conficker remain unclaimed.

Microsoft only offers bounties when the company believes there is a good chance that the money will turn up new evidence and lead to a successful arrest and prosecution, Boscovich says.

"We believe the chances of this reward eliciting good information is really high," he says. "I don't think it makes sense to issue these rewards on a regular basis, because it dilutes the efficacy of the reward itself and desensitizes people to the whole reward structure."

However, the difficulty of finding a way to arrest and prosecute a Russian or Ukrainian citizen -- the likely nationalities of the Rustock operators -- is may make it less likely that an informant will come forward.

The pursuit of the Rustock operators is the latest attempt by the company to shut down a high-profile botnet under the auspices of the Microsoft Active Response for Security (MARS) program. The program aims to use the company's legal and technical teams to create a framework with which to combat cybercrime.

Since the March takedown, spam from the Rustock botnet dropped, although security firms have seen evidence that other spam operators are attempting to take up the slack.

In July, Microsoft noted that Internet addresses that appeared to be infected with Rustock bots dropped by 56 percent since the takedown.