SHANGHAI  China’s detention of four employees of the Anglo-Australian mining giant Rio Tinto on spying charges this week is tied to evidence that they obtained confidential state documents during negotiations with Chinese steel makers over iron ore prices, people with direct knowledge of the case said Friday.

Stern Hu, Rio Tinto’s top iron ore negotiator with China and an Australian citizen, was detained in Shanghai on Sunday on suspicion of spying, stealing state secrets and causing economic harm to the nation, in a case that has rocked the Chinese steel and iron industry. Three other Chinese employees who had some role in Rio Tinto’s iron ore negotiations with Chinese steel mills have also been detained and accused of violating China’s state secrets law.

Now, several other executives in the iron ore trade in China are also under investigation and at least one Chinese executive has been detained, possibly for passing state secrets to the Rio Tinto employees, according to China’s state-run news media and people familiar with the investigation.

Early Friday, Eastday.com, a Web site partly controlled by the Shanghai government, said that the investigation was linked to the pricing of iron ore and that the Chinese authorities were looking into whether Rio employees paid bribes to Chinese officials in exchange for confidential government documents in an effort to gain an edge in negotiations over iron ore contracts.