FORMER foreign minister Alexander Downer believes Melbourne man and suspected Mossad agent Ben Zygier's alleged crimes must have been more serious than simply sharing information with Australian spies. Zygier died in mysterious circumstances in a maximum-security Israeli prison in December 2010 aged 34. It is believed he worked for Israeli spy agency Mossad, but it is unclear why he was arrested and imprisoned. Reports have suggested he was detained for sharing information about Mossad operations with Australian spy agency ASIO. But Mr Downer says that's unlikely to be the whole story. "I suspect that's not the reason he ended up in jail," Mr Downer told ABC radio on Tuesday. I suspect it was something more serious than just sharing information with ASIO. Mr Downer revealed he once raised with Israel's foreign minister allegations that Mossad was using Australian passports to carry out operations abroad. "I did warn them that they shouldn't use Australian passports, or false Australian passports," he said, adding the discussion occurred sometime between 2005 and 2007. Israel "took note" of his concerns, he said. Zygier agreed to be known as "Prisoner X" to protect his family and for national security reasons, Public Security Minister Yitzak Aharonovitch told the Israeli parliamentary assembly today.