A young Slovenian man was taken into custody charged with being the mastermind behind a major computer botnet called Mariposa.

In what the FBI is calling a major breakthrough – after a long investigation by police in Slovenia, as well as the FBI in the United States and Spanish law enforcement – a 23-year-old known as Iserdo was arrested in Maribor, Slovenia. He allegedly wrote the underlying code of the so-called Mariposa botnet that was first identified in Dec. 2008 and reached into 190 countries and more than half of the Fortune 1,000 companies and as many as 40 major banks. It is believed that 12 million computers worldwide were eventually affected.

The apprehension of Iserdo follows five months after police in Spain arrested three men who allegedly operated the Mariposa botnet.

Iserdo’s real name and the charges filed against him have not been released, but the FBI announced that his arrest occurred 10 days ago and that he has been released on bond. Two other colleagues were arrested alongside him, but owing to Slovenian law, their names cannot be revealed. The FBI also stated it expected more arrests to follow, purportedly of some of the buyers of the software package Iserdo allegedly sold.

The investigation crossed national boundaries and was comprised of law enforcement from several countries, as well as a team of more than 100 people, including FBI, members of a specialized botnet investigative team and the Mariposa Working Group, which includes volunteer researchers and private industry experts.

Slovenian police are scheduled to hold a press conference on Friday to release more details on the recent arrests of the three men connected to the Mariposa botnet.

The FBI did not return a call seeking comment on Wednesday.