Additional details cannot be revealed "for reasons of state security," the document said.
Lawyer: I spoke with prisoner shortly before his death
Human rights lawyer Avigdor Feldman said he met with the prisoner a day or two before he allegedly committed suicide.
At the time, Feldman said, the prisoner had not been tried or convicted but was indicted and in pretrial imprisonment.
"His wife asked me to go and see him and look into some legal questions which he had," Feldman said.
Feldman said he met with the prisoner "as a lawyer who was asked not really to represent him but to consider his legal options. It's quite common."
Feldman said that while he was not aware of the details of the cell, "it is supposed to be suicide-proof."
When asked why the prisoner was being held and reports about his dealings with Iran, Feldman answered, "I can't comment unless you want to come and visit me in prison."
One journalist shares his story
Most people worldwide first learned Prisoner X's name as news of the case surged this week.
But journalist Jason Koutsoukis said that an Australian intelligence source first told him about Ben Zygier in October 2009.
The intelligence source, Koutsoukis told CNN, identified Zygier and two other Australian citizens who were also Israeli citizens.
"(The source) suggested to me that they had been involved in a passport scam -- a means to change their identities in Australia and use the new travel documents they obtained to be able to go into countries that were sensitive for Israel," Koutsoukis said.
That issue was a particularly contentious topic between Australia and Israel in 2010, around the time when Zygier was reportedly detained.
Authorities in the United Arab Emirates at the time said that fake Australian passports had been used by three suspects who slipped into a Dubai hotel and killed a Hamas leader. And media and intelligence reports said Israel's Mossad was behind the attack.
Australia's foreign minister summoned Israel's ambassador, saying it was a matter of "the gravest concern."
Australia seeks answers
The latest developments in Prisoner X's case prompted Australian Foreign Minister Bob Carr to request an internal report.
"I'm advised in the form of an interim report that the Australian government was informed in February 2010, though intelligence channels, that the Israeli authorities had detained a dual Australian-Israeli citizen, and they provided the name of the citizen, in relation to serious offenses under Israeli national security legislation," Carr told a Senate committee Thursday.
He did not mention what the alleged "serious offenses" were.
Carr said Australia sought specific assurances from Israel, such as that the detainee would get legal representation of his choosing and that he would not be mistreated.
"At no stage during his detention did the Australian government receive any requests from the individual or his family to extend consular support," Carr said.
"The Australian government was advised through intelligence channels on December 16, 2010, (of) this individual's death on the previous day, and the deceased's family had been notified by Israeli authorities," Carr said.
The Australian Embassy in Tel Aviv assisted in returning the body to Australia, Carr said.
Sharp criticism over secrecy