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 been 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.

The sudden cancellation of the exam caused confusion in South Korea, where many students didn't know of the decision until they arrived at the test sites, according to teachers from private “cram schools” in affluent southern Seoul that specialize in preparing students for the tests.

ACT emails notifying students of the cancellations didn't reach inboxes until nearly 7 a.m., an hour before students were to arrive at test sites, they said.

“It's frustrating for students, including those who had been planning to use the scores for early decision admissions at U.S. schools,” said a cram school teacher who had prepared four students to take the test. She spoke on condition of anonymity out of fear she might anger her clients if she allowed her name to be used.

“Parents have asked whether they should prepare their kids to take the exams in other countries like Japan in the future, because they are worried that the cheating problems could reoccur here, as it had with the SATs,” she said.

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.