Sabelnikov, who is now developing a music recommendation engine called FireHint, won't say exactly what caused him to change his mind.

Microsoft, on the other hand, appears to have somewhat backed down from its contention that Sabelnikov owned, operated and controlled Kelihos. Instead, the company indicates that it agreed with Sabelnikov's claims of innocence.

Richard Domingues Boscovich, assistant general counsel for Microsoft's Digital Crimes Unit, declined to be interviewed on Tuesday, saying the case was closed. But he did provide a statement.

"In the Kelihos case, we were able to identify the developer of the code as well as find out how the code was written and distributed," Boscovich said. "It's important to note that this investigation also revealed how cybercriminals leverage people in the industry to develop code for their illegitimate purposes and that, in some cases, the developer is unaware of how their code will be used upon completion."

Sabelnikov didn't answer questions over his exact involvement with Kelihos. The case, Boscovich said, "allowed us to collect important intelligence and data on how botnets are built."