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MOSCOW (AP) — Ukrainian police say they have uncovered a ring of computer hackers responsible for cyberattacks that targeted universities in the United States and firms South Korea, causing half a billion dollars in damage.

A police statement on Wednesday said the hackers attacked four South Korean companies in 2019 with an encryption virus that blocked internal servers and employees’ computers.

It said the affected companies paid ransom to decrypt the information, but did not name the companies or state the amount paid.

The statement also said the hackers this year targeted personal data and financial records at the Stanford University Medical School, the University of California and the University of Maryland.

The police statement said suspects’ homes were raided and equipment and about 5 million hryna ($185,000) in cash were seized. It did not specify whether any suspects were detained.