According to tabloid Slovenske novice, the FBI contacted the Slovenian police in connection to the case about two weeks ago, urging them to arrange a questioning of three suspects in Maribor.
The suspects were subsequently brought in for questioning, conduced by FBI agents, but none of them was remanded in custody, the paper said.
The Slovenian police have conducted several house searches and interviews themselves. During the investigation, several computers and other pieces of equipment have been confiscated.
Slovenske novice reported today that the suspects were all former students of the Maribor-based Faculty of Computing and IT and were all younger than 25.
They have allegedly sold the virus to Spanish on-line network Mariposa. The virus reportedly infected some 13 million computers world-wide, giving access to information on people's credit cards, on-line banking passwords and other confidential information.
Some 750 major companies and at least 40 banks are said to be infected in the US.
The harmful activities by Mariposa was detected in May 2009 by Canadian security company Defence Intelligence, Spain's Panda Security and US's Georgia Tech Information Security Center.
With the help of the latter two, national and international cyber crime police managed to close down Mariposa in December 2009. The Spanish police arrested the leader, nicknamed Netkairo and hamlet1917, and two of his closest accomplices, Ostiator and Johnyloleante, in early March.
Chief investigator at Panda Security Pedro Bustamante said at the time that the suspects were not particularly skilled hackers, which is why they had hired other cyber criminals to do the dirty work.