SEOUL, South Korea (AP) — Operators of the ACT college entrance exam on Saturday canceled the tests for students in South Korea and Hong Kong at the last moment over what they said was a verified breach of test materials.

The cancellation affected about 5,500 test takers who will receive refunds of test fees, according to ACT spokesman Edward Colby, who said the company believes it was the first time the exam was canceled for an entire country.

The ACT, an Iowa-based nonprofit organization, had planned to administer the tests at 56 different locations in South Korea and Hong Kong on Saturday morning. The ACT decided to cancel the test soon after it received “credible evidence” that the test materials had been leaked, Colby said.

“We are extremely concerned about any activities that could impact the fairness and integrity of the test. When individuals attempt to profit by stealing test materials and selling them, it can hurt thousands of students who did nothing wrong, as it has in this case,” Colby said in an email conversation.

Colby said he couldn’t comment on when and how the test materials might have gotten leaked because the incident is still under investigation. He said the ACT exams will be administered in South Korea and Hong Kong again in September.

South Korea has struggled to clamp down on cheating on standardized English and college admission tests and similar problems have been reported in other Asian countries. The College Board, the New York-based testing firm that oversees the SAT college entrance exams, canceled tests in China and Macau in January over concerns that some students had seen copies of the tests in advance. The College Board was forced to cancel SATs in South Korea in 2013 for similar reasons.