A New York-based consulting firm called the Mintz Group said on Friday that Chinese police raided its offices in Beijing for unclear reasons and arrested all five of the Chinese nationals who worked there. Mintz said the Chinese government has “closed our operations there” and will not allow the company to contact its imprisoned employees.

According to the Mintz Group’s statement, the raid took place on Monday, but as of Friday the company still has not been told why it was targeted. The group said it was prepared to work with Chinese officials to “resolve any misunderstanding that may have led to these events.”

“Mintz Group has not received any official legal notice regarding a case against the company and has requested that the authorities release its employees,” the statement said.

“Our top priority is the safety and well being of our colleagues in China, where we have retained legal counsel to engage with the authorities and support our people and their families,” it added.

Various international media organizations reported on Friday that Chinese officials would not comment on the case. 

The Mintz Group specializes in providing corporate services such as background checks and “due diligence.” The company has 18 offices worldwide, but only one in China. 

A Mintz executive told the Wall Street Journal (WSJ) that the company has no idea which agencies of the Chinese government took its employees prisoner or what law they might be charged with violating. The missing employees have reportedly not been allowed to contact their families.

One theory behind the mysterious raid, advanced to Reuters on Friday by a source in the American business community, was that China wanted to send a “remarkable signal” that it wants foreign money, business opportunities, and technology but will not allow any oversight of its business environment, including the sort of investigative services Mintz provides.

“Red alerts should be going off in all boardrooms right now about risks in China,” said the source, who insisted on remaining anonymous for fear of angering the regime in Beijing.

“The foreign business community needs due diligence in order to conduct safe business, to pick the right partners and the right hires, to invest in the right companies without losing their shirt … But Beijing has made it impossible to do this,” agreed British corporate investigator Peter Humphrey, who was arrested by the Chinese in 2013 and spent two years in jail for conducting due diligence work on behalf of a pharmaceutical company.

The WSJ noted that an early sign of trouble for Mintz came in January when a Singaporean national working in the Beijing office was inexplicably denied permission to leave China on a holiday trip for Lunar New Year. The female employee was never charged with a crime, but it took weeks for the Singaporean embassy in Beijing to negotiate permission for her to leave the country.

“The corporate investigations business is tolerated by Chinese authorities but has been targeted for crackdowns during leader Xi Jinping’s decade in office. In addition to tightly controlling media and the internet, Chinese authorities increasingly limit access to business data, including by labeling it private information or national property,” the WSJ noted.

Another possibility is that Mintz was targeted because the partner in charge of its Asia operations is Randal Phillips, a 28-year veteran of the CIA and its former chief representative in China. Phillips has given congressional testimony about Chinese influence operations in the past, but no particular recent action of his leaps out as a reason why China might abruptly decide to shutter his company’s Beijing office.

The New York Times (NYT) on Friday noted that the China Development Forum, “an annual gathering of multinational chief executives and senior Chinese government officials,” is scheduled to begin tomorrow. The Mintz raid might have been intended to send a message to the attendees, who include Apple CEO Tim Cook. 

“Few other prominent American executives are scheduled to attend or speak at the forum this year. Many American companies have become worried about getting caught between pressure from the Chinese government to express support, and pressure from Congress to show that they are standing up to Beijing on issues like human rights, and have gone silent,” the NYT observed.