Taiwan’s Mainland Affairs Council, in a statement to Reuters on Thursday, said the four employees had been detained in China’s Zhengzhou, the capital of Henan province that is home to a major Foxconn plant assembling Apple’s (AAPL) iPhones, on suspicion of the equivalent of “breach of trust.”

“The circumstances of this case are quite strange,” it said.

The case may be connected to corruption and abuse of power by a small number of Chinese security officials, the council added, without giving details.

Foxconn declined to comment.

The council said Foxconn had stated the company had “suffered no losses and that the four employees had done nothing to harm the company’s interests.”

“This has seriously damaged the confidence of companies. We call on relevant departments on the other side of the Taiwan Strait to investigate and deal with it as soon as possible,” it said.

In June, Taiwan’s government raised its travel warning for China, telling its citizens not to go unless absolutely necessary, following a threat from Beijing to execute those deemed “diehard” Taiwan independence supporters.