The document is 15 pages long, and among other things, it describes procedures for fabricating hemispheres of uranium metal – key components for a nuclear weapon.
International inspectors since 2005 have known Iran had this paper in its possession, but they got a copy of it only last November. Tehran's explanation for its existence? Iranian officials say they received it unbidden in a pile of instructions that came with uranium-enrichment devices from Pakistan.
US officials – plus many experts outside government – don't buy this story. The paper, they say, is just one of a number of publicly known items related to Iran's nuclear program that have yet to be explained and appear suspicious.
"There are still outstanding issues, and some of them are pretty serious," says Sharon Squassoni, a senior associate in the nonproliferation program at the Carnegie Endowment for International Peace in Washington.
According to a recent US National Intelligence Estimate, Iran halted a clandestine nuclear-weapons program four years ago but continues to develop uranium-enrichment technology applicable to weapon development.
Iran has long insisted that its nuclear efforts are peaceful and intended to produce only electric power. According to Iranian officials, the latest report of the United Nations' nuclear watchdog, the International Atomic Energy Agency (IAEA), shows they have resolved all outstanding questions about their program.
Evidence to the contrary is fabricated, said Mohammad Khazee, Iran's ambassador to the UN, on Monday. He warned that a US push for a further round of sanctions on his nation could have negative repercussions.
"Iran has been cooperating with the IAEA more seriously and more sincerely and beyond its obligation, so somebody should ask [Secretary of State Condoleezza Rice] where these questions come from," said Ambassador Khazee.
One place they might come from is the IAEA itself. The watchdog group's latest report on the Iranian program, dated Feb. 22, does note that Tehran has answered some outstanding questions. Suspicious uranium particle contamination found by the IAEA on some equipment at a technical university may have come from a repair technician who had been testing parts from centrifuges used in fissile-material enrichment.
"The Agency considers this question no longer outstanding at this stage," says the IAEA report.
But some major questions remain unanswered or in dispute.
Take the 15-page paper on the fabrication of uranium metal. Iran says it didn't ask for the information and just received it when it took delivery of centrifuge material from the network of Pakistani weapons proliferator A.Q. Khan.
Given that uranium hemispheres are the heart of a nuclear weapon, Iran immediately should have disclosed the document's existence to the IAEA, says Ms. Squassoni of the Carnegie Endowment. Inspectors have asked Pakistan for its side of the story, but as to getting a response, "don't hold your breath," she says.
Another big issue relates to intelligence information given to the IAEA by the United States or other UN member nations. Some of the data was contained in a so-called "laptop of death" used by an Iranian official and reportedly obtained by German intelligence.
One of these documents is a sheet that contains information about the conversion of uranium dioxide into uranium tetrafluoride – an intermediate step in the production of uranium metal. This document makes reference to the leadership of another project dealing with the production of a missile reentry vehicle, according to the IAEA.
This paper did not come from US intelligence or the purloined laptop.
"It's very significant, actually," says David Albright, a former weapons inspector and current president of the Institute for Science and International Security (ISIS) in Washington.
Iran says this paper is a fake, as are others dealing with possible weapons R & D that IAEA inspectors asked for explanation on. "The bottom line is, stuff remains murky," says Mr. Albright.
A report by ISIS notes that another interesting item released by the IAEA is data indicating that Iran's P-1 centrifuges – spinning cylinders used to enrich uranium gas – aren't working very well. Iran is instead beginning to turn to a second-generation centrifuge, designated IR-2. This model is an Iranian adaptation of a European design purloined by Dr. Khan.
IAEA data indicate that the new machines could produce up to 2.5 times as much material per centrifuge as their predecessors.