CAIRO (Reuters) - Egyptian prosecutors ordered the detention of four relatives of the head of an alleged criminal gang which police said was found with belongings of murdered Italian student Giulio Regeni, judicial sources said on Saturday.

The gang leader's wife, brother, sister, and brother-in-law were held, accused of concealing a criminal and hiding stolen goods, the sources said. Police have said they found Regeni's possessions at a house belonging to the sister.

A judicial source told Reuters that local prosecutors dealing with the gang case requested that senior prosecutors handling Regeni's death add the questioning of the four relatives to their case file.

On Thursday, Egypt's interior ministry said the gang, whose four members were all killed in a police shootout, had Regeni's bag and passport in their possession. The group specialised in impersonating police and kidnapping foreigners, it said.

But the statements of the police and the ministry have failed to satisfy Italian investigators, politicians and Regeni's family.

Rome's chief prosecutor, Giuseppe Pignatone, said on Friday that the investigation into Regeni's murder would continue.

The broken body of the 28-year-old Cambridge University student, who was researching independent labour unions, was found at the side of a motorway on the outskirts of Cairo in February.

Human rights groups have said torture marks on Regeni's body indicated he died at the hands of Egyptian security services, an allegation the government has strongly denied.

The case has caused friction between Egypt and Italy, though it is unlikely to lead to a permanent rift because of the two countries' economic ties and Egypt's strategic role in the Middle East.

(Reporting by Haitham Ahmed; Writing by Ahmed Aboulenein; Editing by Nerys Avery)