WASHINGTON — Federal agents over the weekend secretly seized control of two computer networks that hackers used to steal millions of dollars from unsuspecting victims. In doing so, the Justice Department disrupted the circulation of two of the world’s most pernicious viruses and turned a 30-year-old Russian computer hacker into a most-wanted fugitive.

The strike, coordinated with the European authorities, was aimed at malware called GameOver Zeus, which is known to steal bank information and send it to overseas hackers, and CryptoLocker, which burrows into computers and encrypts personal data. The hackers then demand a ransom to unlock the files.

“By the time the victims learned that their computers had been infected, it was far too late,” Leslie R. Caldwell, the assistant attorney general in charge of the criminal division, said Monday.

Together, the Justice Department estimates, the two malicious programs have infected between 500,000 and a million computers and cost people more than $100 million in direct and indirect losses.