Italy has opened an investigation into the suspected "kidnapping" of the only survivor of a cable car accident in May.

       Six-year-old Eitan Biran was flown to Israel on Saturday by his maternal grandfather on a private jet.

       He was the only person pulled alive from the wreckage when a cable car fell near Lake Maggiore on May 23 in Stresa, a seaside resort in Piedmont.

       Fourteen people -- including Eitan's parents, younger brother, and two great-grandparents -- were killed. Eitan was seriously injured and spent several weeks in hospital in Turin.

       Since his release, his paternal family in Italy and his maternal family in Israel have been fighting over his custody.

       Eitan's paternal aunt Aya Biran-Nirko, a resident of the northern province of Pavia, has legal guardianship over him.

       His maternal grandfather had reportedly come for a visit but taken the child to Israel.

       The Pavia prosecutor's office opened an investigation into the kidnapping on Sunday, according to the Ansa agency.

       On Tuesday, Israeli police said they had questioned the six-year-old boy's grandfather at his home in Petah Tikva on suspicion of involvement.

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       Eitan had been entrusted to his paternal aunt by the Turin juvenile court, while his mother's family had visitation rights. The Italian court had also ordered the return of his passport, which was in the possession of his maternal family.

       In an interview with Israeli radio 103 broadcast on Sunday, his maternal aunt, Gali Peleg, gave her version of his return to Israel.

       "We didn't kidnap him (...) we brought him home. We had to do this because we had no information about Eitan's physical and mental health," she explained.

       "We brought him home, that's what his parents wanted, they were supposed to come back and they postponed it because of the coronavirus but when they were here a few months ago, we had talked with them about going back," she assured.

       Eitan's parents had been living in Italy for several years where his father was a medical student. Their youngest son was also born there in 2019.

       The president of the Jewish community in Milan condemned "an act that violates Italian and international laws".

       "We hope that the case will be resolved as soon as possible in accordance with the decision of the juvenile court," Milo Hasbani added in a statement.