WASHINGTON—U.S. and foreign law-enforcement agencies in recent days broke up a network of hijacked computers that authorities say were used by hackers to tap into hundreds of thousands of bank accounts and steal at least $100 million, U.S. officials said Monday.

Authorities in the U.S. and Europe also targeted the infrastructure behind software that infected more than 200,000 computers around the world since September 2013 and threatened users with the destruction of their data unless they paid a ransom of a few hundred dollars.

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