Hong Kong's anti-graft agency has arrested two of the world's wealthiest tycoons on suspicion of bribery.

Billionaire brothers Thomas and Raymond Kwok control Hong Kong's biggest real estate empire and have built some of the most famous towers on the city's skyline. They were detained for questioning Thursday, as was a former top government official, Rafael Hui, who was accused of “misconduct in public office.”

The Kwok brothers were recently ranked by Forbes magazine as No. 27 on its list of the world's billionaires, with a family fortune of more than $18 billion.

The city's Independent Commission Against Corruption made the arrests, although no charges were filed.

The Kwok brothers are joint chairmen of Sun Hung Kai Properties, which said the investigative agency issued a search warrant for its offices and that it was forced to provide “certain information” connected to the case.