A recently foiled botnet operation has turned out to be 15 times larger that police initially thought.

On further investigation, authorities found that the operation had put about 1.5 million computers and servers under its control. The crime ring was thought to have created a botnet of 100,000 systems, which they claimed was the largest ever detected.

A botnet is a collection of computers infected with a computer worm which puts the system under the worm creator's control without the owner's knowledge.

Botnets are commonly used to host illegal web content such as child pornography, to send out spam or to launch distributed denial of service (DDoS) attacks.

Dutch police earlier this month made three arrests in the case. They have now extended the pre-trial imprisonment of two of the suspects, the 19 year-old prime suspect and a 22 year-old accomplice.

A third member of the group has been released for "personal reasons", according to Dutch IT news website Webwereld. 

The 22 year-old is likely to face additional blackmail charges for launching a DDoS attack against an unnamed US corporation. The group has already been linked to another DDoS attack against a corporation in the US.

Other charges include computer hacking, destruction of automated networks and installing adware and spyware. 

The Dutch public attorney said in a press release that he expects to make additional arrests in this case.

Police apprehended the trio on 4 October, confiscating computers, cash and a sports car during searches of the suspects' homes.