The UN is investigating how vials of an apparently harmless substance were mistaken for toxic chemicals, sparking a major alert at a New York office.
The vials were found in an office building near the UN's headquarters
The containers, first thought to hold the chemical agent phosgene, were discovered at the UN's weapon inspection agency last month.
Initial tests suggested it was actually a harmless solvent, although officials stressed the analysis was incomplete.
It is thought the vials might have been mislabelled when they were stored.
UN spokeswoman Michele Montas said the episode raised "security issues".
Confirming an inquiry would take place, she said: "One has to know why it happened so it won't happen again."
The substance, found at the UN Monitoring, Verification and Inspection Commission (Unmovic), was said to have been removed from Iraq during inspections of chemical facilities a decade ago.
The UN said the vials had been found by inspectors as they cleared out an office near the UN's headquarters on 24 August.
Officials did not disclose the find until last week, when a UN spokeswoman said the material "could be potentially hazardous", but has not posed any immediate risk.
The metal and glass vials, stored in a sealed plastic bag, were discovered in the process of emptying Unmovic's office building in Manhattan.
"The only information we have of the contents of that bag is from an inventory of a 1996 inspection," the UN's Marie Okabe said at the time.
She said the inventory "indicates that one of the items may contain phosgene, an old generation chemical warfare agent".
Phosgene, a yellow-green gas, was widely used during World War I as a choking agent. The Russian military has also claimed that it has been used in Chechnya.
Unmovic's inspectors were withdrawn from Iraq in March 2003 and in June this year the UN decided to end its mandate.