Information about the case emerged briefly in June 2010, when the Israeli news site Ynet reported on the existence of a prisoner -- identified only as Prisoner X -- whose crimes were unknown. The report was mysteriously removed from the site shortly after it was posted, apparently under pressure from Israel's military censor. The censor has authority to block or delete reports deemed threatening to national security.
Ynet then reported on December 27, 2010, that a prisoner had committed suicide while in solitary confinement two weeks earlier. That report was also quickly removed.
The Israeli censor's office declined comment.
The ABC reported that Zygier moved from Australia to Israel in 2000, was 34, was married to an Israeli woman and was the father of two young children. It reported that his incarceration was top secret but did not say why he had been arrested.
It said he hanged himself in a cell that had been specially designed for Yigal Amir, the Jewish ultranationalist who in 1995 assassinated then-Prime Minister Yitzhak Rabin.
Australia's foreign affairs department would not comment on what it described as the ABC's "speculation" on Allen's links to Mossad.
Israeli TV speculated that Zygier was imprisoned after committing some kind of act of treason.
Israeli Channel 10 TV reported that in 2009, Australian intelligence officers interrogated Zygier when he was in Australia, questioning him about trips to Iran, Lebanon and Syria.
The TV report alleged that the matter was leaked to an Australian reporter, who phoned Zygier and questioned him about his alleged Mossad links. He reportedly denied the allegations. The TV report said some time later, he was put in jail in Israel, and six months later he was found dead in his cell.
In Israel, media were prevented from reporting many details of the Zygier case because of a court-imposed gag order.