A secret assessment of Iran's nuclear weapons program, leaked to the press over the weekend, is offering a window on how far Iran has advanced toward building a nuclear bomb – and how much is still unknown.

The information is, in same cases, remarkably precise.

"Project 110" is the code name of Iran's secret effort to produce a nuclear warhead capable of fitting inside the nose cone of a Shahab 3 missile.

"Project 111" is corresponding work aimed at reshaping the space inside the nose cone itself.

In addition, Iranian scientists already may have developed sophisticated bridgewire detonators and associated high-voltage firing systems necessary to initiate an implosion warhead's explosion.

Yet the International Atomic Energy Agency (IAEA) report has been the subject of fierce debate within the IAEA itself.

US officials say they continue to stand by the conclusion of the US intelligence community that Iran halted nuclear weaponization work in 2003.

The report appears to be based on data smuggled out of Iran by the wife of an Iranian who was recruited by German intelligence, according to the Institute for Science and International Security (ISIS), which has posted excerpts from the report on its website.

The smuggled information contains large quantities of test data that would be difficult to fake convincingly, according to experts consulted by ISIS.

Still, the internal IAEA study that draws on this information has critics within the agency. Some IAEA officials worry that it goes too far in concluding that Iran now has "sufficient information to be able to design and produce a workable implosion nuclear device".

And some experts outside government caution that the parts of the report so far leaked to the public don't necessarily support that conclusion.

The report does not have much in the way of timelines, for instance, meaning it is possible that Iran may have had a "Project 110" at one time, which has since been halted, as the US holds.

The IAEA report "could be consistent" with US intelligence conclusions, says Greg Thielmann, a former US intelligence official.

It may also be important to remember that in general a nuclear weapons project and a successful nuclear weapons project are two different things.

Drawing up and producing a nuclear warhead capable of fitting on a missile is a difficult challenge.

"If it were easy, handing someone the design of a warhead would be tantamount to giving them a nuclear capability. That's not the case," says Mr. Thielmann, who is now a senior fellow at the Arms Control Association.

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