Microsoft says it has concluded its civil case against the Rustock botnet operators and referred evidence to the FBI.

In a statement, Microsoft Malware Protection Centre spokesperson Jeff Williams said that while the case is now with the FBI, the company is still offering $250,000 (£155,000) for information that leads to the arrest and conviction of Rustock's operators.

Williams said: “We will continue to work with internet service providers and computer emergency readiness teams to clean infected computers using the telemetry we receive from having control of Rustock's command and control domains.”

Microsoft offered the reward following the takedown of the Rustock botnet in March.

Richard Boscovich, senior attorney of the Microsoft digital crimes unit, said: “While the primary goal for our legal and technical operation has been to stop and disrupt the threat that Rustock has posed for everyone affected by it, we also believe the Rustock bot-herders should be held accountable for their actions.”

Following the takedown of the botnet, reported a global drop in spam of 33 per cent.