Leon Keder, a spokesman for the Slovenian police, did not identify the suspect. Keder told The Associated Press the man was released after police made sure that he could not tamper with evidence or leave Slovenia, but offered no details pending an investigation.

The FBI told The AP in Washington that a 23-year old Slovene known as Iserdo was picked up in Maribor in northwestern Slovenia 10 days ago, after lengthy investigation by Slovenian police, FBI and Spanish authorities.

His arrest comes about five months after Spanish police broke up the massive cyberscam, arresting three of the alleged ringleaders who operated the Mariposa botnet, which stole credit cards and online banking credentials. The botnet -- a network of infected computers -- appeared in December 2008 and infected hundreds of companies and at least 40 major banks.

Botnets are networks of PCs that have been infected by a virus, remotely hijacked from their owners, often without their owners' knowledge, and put into the control of criminals.

The Mariposa botnet, which has been dismantled, was easily one of the world's biggest. It spread to more than 190 countries, according to the researchers who helped take it down after examining it in the spring of 2009.

Jeffrey Troy, the FBI's deputy assistant director for the cyber division, said Iserdo's arrest was a major break in the investigation.

On Wednesday, the FBI also identified, for the first time, the three individuals arrested in connection with the case in Spain: Florencio Carro Ruiz, known as "Netkairo;" Jonathan Pazos Rivera, known as "Jonyloleante;" and Juan Jose Bellido Rios, known as "Ostiator.

They are being prosecuted for computer crimes. Officials said the Mariposa botnet from Spain was the largest and most notorious.

In Ljubljana, Keder said "other suspects" were detained and interrogated along with the chief suspect, but offered no further details until a news conference planned for Friday.

Slovenian media have linked three former students of the Maribor Faculty of Computing and IT to the case, reporting that they were recently detained and interrogated by police and FBI officials, who confiscated their computer equipment.

The FBI's Troy said more arrests are expected and are likely to extend beyond Spain and Slovenia, targeting additional operators who allegedly bought the malware from Iserdo.

Mariposa is the Spanish word for "butterfly." Iserdo, read backwards, means "salvation" in Slovenian.

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