Police believe the young man was working with two British men to create predatory computer programs called trojan horses. Much like their mythic namesake, trojans are malicious programs that are disguised as something benign, or are perhaps inserted into legitimate software. If accidentally uploaded, they can grant the hacker control of the infected computer.
The officer in charge of the Finnish side of the criminal investigation, Kirsi Kujala, is still unsure that they've caught all the culprits, noting that there could very well be more Finns involved. The investigation is ongoing.
The Finnish suspect and his colleagues apparently were able to create such programs that bypassed the normal virus protection software. The ramifications of such widespread cyber-attacks are worrisome especially for businesses. Police say that the group infected potentially millions of computers worldwide, although British companies were specifically targeted.