Criminals have built a massive "botnet" network of hacked computers that has infected an estimated 1.9 million computers worldwide, including systems within U.S. and U.K. government networks, security vendor Finjan reported Tuesday.

A group of six criminals has run the operation since February, controlling the botnet from a server located in the Ukraine, Finjan said. Using a network of Web sites, they have installed their malware by tricking victims into viewing malicious Web code that then installs the malware.

Nearly half of the computers hacked by the gang are located in the U.S., but they have also hit a sizeable number of machines in the U.K., Canada, Germany and France, Finjan said.

Finjan researchers found "compromised computers in 77 government-owned domains ... from the U.S., U.K., and various other countries," the company said in a statement.

Finjan has contacted institutions that were hacked by the gang and is working with law enforcement on the issue, the company said.