Cairo – Egyptian prosecutors ordered on Saturday the detention of four people in relation to the murder of Italian student Giulio Regeni, a prosecution official said. Two of them are the wife and a sister of a gang leader whom police have linked to the brutal murder of Regeni, whose mutilated body was found more than a week after his disappearance in Cairo on January 25.They had been arrested in the sister's apartment, where police discovered a bag with Regeni's passport and wallet.The other two are the brother-in-law and brother of the alleged gang leader, who was killed in a shoot-out with police along with three other alleged criminals.The four suspects are accused of concealing a crime and being in the possession of stolen material, the official said, adding they were taken into custody for four days.Egyptian police announced on Thursday that they discovered Regeni's passport and wallet in the home of the leader's sister, hours after the shoot-out.Italy has cast doubt on the suggestion that the gang members – who allegedly posed as police to extort foreigners and Egyptians – were behind Regeni's murder."Italy insists: we want the truth," wrote Italian Foreign Minister Paolo Gentiloni on his Twitter account, while prosecutors in Rome rejected the latest conclusions of the Egyptian probe.Italian media and western diplomatic sources in Cairo have voiced suspicions that Egyptian security services kidnapped and tortured to death the 28-year-old Cambridge University graduate student.Rome prosecutor Giuseppe Pignatone said in a statement that "details communicated so far are not satisfactory to shed light on the death of Giulio Regeni. Investigations must therefore continue."According to Italian government sources, Prime Minister Matteo Renzi has promised Regeni's parents that Rome will continue to put pressure on Egypt to establish the full truth behind his death.Quoted by Italian press, the parents have said they were "injured and bitter" at Egyptian authorities' latest attempt to explain their son's death.Regeni had been researching labour movements in Egypt, a sensitive topic, and had written articles critical of the government under a pen name.In a statement late on Friday, the Egyptian interior ministry said it was investigating the gang's links to Regeni's murder."The investigation apparatus is continuing, in coordination with the Italian security team, in its efforts to examine the gang's links, and the circumstances of the crimes and the areas in which they occurred," the ministry said.