Seeking to mend ties with the West, Libya's Muammar Gaddafiagreed in 2003 to abandon efforts to acquire nuclear, chemical and biological weapons -- a move that brought him in from the cold and helped end decades of Libyan isolation.
A six-month popular insurgency has now forced Gaddafi to abandon his stronghold in the Libyan capital but continued gunfire suggests the rebels have not completely triumphed yet.
Olli Heinonen, head of U.N. nuclear safeguards inspections worldwide until last year, pointed to substantial looting that took place at Iraq's Tuwaitha atomic research facility near Baghdad after Saddam Hussein was toppled in 2003.
In Iraq, "most likely due to pure luck, the story did not end in a radiological disaster," Heinonen said.
In Libya, "nuclear security concerns still linger," the former deputy director general of the International Atomic Energy Agency (IAEA) said in an online commentary.
Libya's uranium enrichment program was dismantled after Gaddafi renounced weapons of mass destruction eight years ago. Sensitive material and documentation including nuclear weapons design information were confiscated.
But the country's Tajoura research center continues to stock large quantities of radioisotopes, radioactive waste and low-enriched uranium fuel after three decades of nuclear research and radioisotope production, Heinonen said.
Refined uranium can have civilian as well as military purposes, if enriched much further.
"While we can be thankful that the highly enriched uranium stocks are no longer in Libya, the remaining material in Tajoura could, if it ended up in the wrong hands, be used as ingredients for dirty bombs," Heinonen, now at Harvard University, said.
"The situation at Tajoura today is unclear. We know that during times of regime collapse, lawlessness and looting reign."
A so-called dirty bomb can combine conventional explosives such as dynamite with radioactive material.
Experts describe the threat of a crude fissile nuclear bomb, which is technically difficult to manufacture and requires hard-to-obtain bomb-grade uranium or plutonium, as a "low probability, high consequence act" -- unlikely but with the potential to cause large-scale harm to life and property.
But a "dirty bomb," where conventional explosives are used to disperse radiation from a radioactive source, is a "high probability, low consequence act" with more potential to terrorize than cause large loss of life.
"There are a number of nuclear and radiological materials at Tajoura that could be used by terrorists to create a dirty bomb," said Mark Fitzpatrick, a director at the London-based International Institute for Strategic Studies think-tank.
There was no immediate comment from the IAEA on the Tajoura facility. A document posted on the IAEA's website said it was a 10 megawatt reactor located 34 km (20 miles) east of the Libyan capital.
The Vienna-based U.N. agency has been involved in technical aid projects in Libya, including at Tajoura.
Heinonen said Libya's rebel Transitional National Council would need to be aware of the material at Tajoura. Once a transition takes place it should "take the necessary steps to secure these potentially dangerous radioactive sources."
Fitzpatrick said the looting that occurred at Iraq's Tuwaitha center "should stand as a lesson for the need for nuclear security precautions in the situation today in Libya."
(Editing by Mark Heinrich)