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 .
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
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 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," said Italian Foreign Minister Paolo Gentiloni on his Twitter feed late , while prosecutors in Rome rejected the latest conclusions of the Italian probe.
Italian media and western diplomatic sources in Cairo have voiced suspicions that Egyptian security services kidnapped and tortured to death the Cambridge University graduate whose mutilated body was found in Cairo in January.
Regeni disappeared on in central Cairo. His body was found nine days later on the side of a motorway, badly mutilated and showing signs of torture.
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, premier Matteo Renzi has promised the parents of the young student that Rome will continue to put pressure on Egypt to establish the full truth behind his death.
"Italy will never content itself with anything less than the truth, the
whole truth, without shadows or mystery," a source within the prime minister's office said.
Quoted by Italian press, Regeni's parents said they were "injured and bitter" at Egyptian authorities' latest attempt to explain their son's death.
Regeni, 28, had been researching labour movements in Egypt, a sensitive topic, and had written articles critical of the government under a pen name.
The incident threatened relations between Egypt and Italy, a strong supporter of President Abdel Fattah al-Sisi whose security services have been accused of abusing dissidents.
In a statement late , 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