Four Rio Tinto staff being held in China on charges of industrial espionage could face prison sentences after their case was handed over to prosecutors today.Australia's Department of Foreign Affairs and Trade say they've been told by the Chinese authorities that the investigation phase is over and the case is now with the Shanghai People's Procuratorate. It is here where the decision whether or not to bring the four workers at Australian miner Rio to trial will be made.'We are not in a position to say how long this phase of the case will take, and are not prepared to speculate about the outcome,' said officials in Canberra.Australian national Stern Hu was arrested last July along with three Chinese colleagues and accused of stealing state secrets.Some observers believe the arrests are linked to Rio Tinto's rejection of a controversial $19.5bn refinancing deal with Chinese state-run firm Chinalco last June in favour of a $15.2bn rights issue and a merger of its iron ore assets with those of BHP Billiton.It also coincided with the failure of annual iron ore contract talks.'This transfer is the next stage in a continuing legal process under Chinese law,' said Rio Tinto Iron Ore unit boss Sam Walsh on Monday. 'It would not be appropriate for the company to comment any further at this point in the case other than to reaffirm our hope that matters proceed in an expeditious and transparent manner.'Chinese law dictates that a decision by prosecutors must be made in about a month's time, although it could take several months if they ask for supplementary investigations.