Spanish officials are calling it the world’s biggest network of virus-infected computers. That’s right — 13 million computers were hacked and infected with a program that allowed for the theft of personal and financial data of unwitting citizens worldwide, in what investigators have termed the “Mariposa botnet.”

The culprits? Three Spaniards, ages 31, 30 and 25 who were arrested last week in Spain’s northern Vizcaya province. During a search of their homes and computers, police found personal information from more than 800,000 users.

The suspects “copied personal and financial data of individuals, companies and official institutions in more than 190 countries,” the Civil Guards’ statement said. In addition to gaining illegal access to personal and financial information, the virus would have permitted those controlling the system to mount a large cyberattack from the infected computers, a U.S. official said.

Apparently, the hacking was first detected in May by Defence Intelligence, a Canadian firm that develops software to monitor incoming and outgoing transmissions of every computer on a corporate network.

The botnet [or, a network of computers infected with a virus that can be controlled remotely without owners’ knowledge] included infected machines inside the offices of more than 40 major banks and a vast majority of the top companies on the Fortune 1000 index. Since its creation, Defence Intelligence has identified and helped to protect its customers against dozens of botnets and malicious software programs — with Mariposa botnet being the jewel in its crown.

Authorities are searching for a fourth suspect in this massive cybercrime. “Juan Salon of the Spanish Civil Guard’s cybercrime unit told reporters Wednesday investigators have learned the suspect’s Internet handle and that this person might be Venezuelan.”