The U.S. corporate due diligence firm Mintz Group claimed Chinese officials stormed its Beijing office and arrested five local personnel, raising concerns among foreign corporations in China as it holds an international economic event.

After months of diplomatic tensions, including the U.S. military downing a suspected Chinese spy balloon in February and the president of Taiwan, the self-governed island Beijing claims as its own, transiting the U.S. next week, the raid and detentions have strained Sino-U.S. ties.

While China hosts an international business event, foreign firms in China are concerned after Chinese officials raided Mintz Group’s Beijing headquarters and arrested five local employees.

After months of diplomatic tensions, the raid and detentions have strained Sino-U.S. relations, including the U.S. military downing a suspected Chinese spy balloon in February and Taiwan’s president visiting the U.S. next week.

Mintz Group claimed it had not received any official legal notification regarding a lawsuit against the firm and urged that the authorities release its workers.

Friday’s Chinese foreign ministry spokesman Mao Ning was unaware of this matter. Beijing’s public security officials declined to comment.

A source at the New York-based firm told Reuters on condition of anonymity that the company’s local legal counsel indicated the raid occurred on the afternoon of March 20 and that the employees were being kept incommunicado outside of Beijing.

According to Mintz Group’s website, Beijing is its lone mainland China office. The website advertises background checks, fact collecting, and internal investigations. Media reports include the NFL, New York City, and The Beatles as clients.

Mintz employs hundreds in 18 offices worldwide. The firm’s website lists Randal Phillips, its Asia operations leader stationed outside of China, as the CIA’s former main representative in China.

Phillips spent years in Beijing after the CIA. He was unavailable for comment, and Reuters found no connection to the event.

The search and detentions come as Beijing prepares to host the three-day China Development Forum on Saturday, with more than 100 abroad attendees from multinationals and international organizations.

One U.S. business community member told Reuters the Mintz Group episode sent an “extraordinary signal” that Beijing wants foreign money and technology but won’t allow reputable U.S. corporations to undertake due diligence on Chinese partners or the business climate.

“Red alerts should be going out in all boardrooms right now regarding risks in China,” said the person, who requested anonymity due to the topic’s sensitive nature.

China welcomes international commerce and investment but prioritizes security above progress.

According to an American Chamber of Commerce in China poll, U.S. companies operating in China are becoming more negative about their prospects in the world’s second-largest economy.

Rising U.S.-China relations were the top business challenge for two-thirds.

Chinese officials have cited Western due diligence firms. For example, in 2013, British corporate investigator Peter Humphrey and his American wife Yu Yingzeng, who owned risk consultancy ChinaWhys, were arrested for GSK work.

An anonymous employee of another foreign due diligence business in China said it was unclear if Mintz Group was a unique case or part of a wider campaign.

“Some organizations may now opt to exercise extra caution while undertaking due diligence tasks in China,” the employee said.