Over the past year, the Trump family has zoomed around the world signing one new real estate development after another, often involving foreign governments, raising a litany of ethical concerns.

But only one of those has led to a publicly announced criminal investigation of local officials. And the inquiry came after a plucky group of cultural preservationists in Serbia stood up to their own government and, by extension, the close relatives of the powerful American president.

President Trump’s son-in-law, Jared Kushner, has a deal with the Serbian government to build a half-billion-dollar hotel and apartment complex in the center of the capital, Belgrade. The project also involves the Trump Organization, run by the president’s sons Eric and Donald Jr., as the luxury hotel will bear the Trump brand.

In November, one week after Mr. Trump won re-election, the Serbian government greased the skids by declaring that the site — a bombed-out building that serves as an icon to Serbians’ suffering during a 1999 conflict — was no longer considered a culturally protected asset. That paved the way for the Trump family project.

Dozens of architects and cultural historians at the state-run Republic Institute for the Protection of Cultural Monuments cried foul, accusing the government of violating the law. Several days after the government’s decision, they fired off a letter saying the property’s status as an “immovable cultural property” could be revoked only if a team of the institute’s experts approved it. And they hadn’t.

“From the beginning, we knew it was a political decision,” said Estela Radonjic Zivkov, the institute’s former deputy director. She said she was pressured by state intelligence officers not to challenge the government on this case, a clear sign of Serbian leaders’ intense interest in the project. She did so anyway.