WASHINGTON, July 28 (UPI) -- The FBI said Wednesday an international investigation of the Mariposa Botnet, a "zombie" network formed by a computer virus, is showing progress.

The two-year investigation was conducted with the Slovenian Criminal Police and the Spanish Guardia Civil.

The Mariposa Botnet was built with a virus called "Butterfly Bot," the FBI said. The botnet was used to steal passwords for Web sites and financial institutions, steal computer users' credit card and bank account information, launch denial of service attacks and spread viruses.

The FBI said experts estimated the Mariposa Botnet may have infected 8 million to 12 million computers.

"In the last two years, the software used to create the Mariposa botnet was sold to hundreds of other criminals, making it one of the most notorious in the world," FBI Director Robert Mueller said. "These cyber intrusions, thefts, and frauds undermine the integrity of the Internet and the businesses that rely on it; they also threaten the privacy and pocketbooks of all who use the Internet."

The Spanish Guardia Civil arrested three suspected Mariposa Botnet operators in February, the FBI said. The three are being prosecuted in Spain for computer crimes.

Last week, the Slovenian Criminal Police identified and arrested the Mariposa Botnet's suspected creator, a 23-year-old Slovenian citizen known as "Iserdo."