UNITED NATIONS — Nearly half a decade after the last United Nations weapons inspector left Iraq, an inspection team said yesterday that a potentially deadly chemical agent was discovered in its own Midtown offices.

A suspicious soda can-size bag found in the offices of the U.N. Monitoring, Verification, and Inspection Commission last Friday was filled with plastic and glass vials and tubes containing grams of phosgene, a choking agent used as chemical weapon in the World War I era and during the 1980–88 Iran-Iraq War.

Phosgene is a toxin that, once released, causes human lungs to collapse, a UNMOVIC scientist, Svetlana Utkina, told a dozen reporters gathered yesterday in a small U.N. briefing room. But the chemical is neither stable enough to be used in modern warfare nor a mass killer. If a container with grams of phosgene “would be opened here and evaporate here, probably around five people would get severe problems, and a couple of people would be dead,” Ms. Utkina said.

The substance was discovered by an unidentified UNMOVIC employee last Friday and put inside a safe bag, according to an agency spokesman, Ewen Buchanan, who added that the area where the bag was stored was secured and tested by the agency’s scientists.

They found the environment to be safe enough for employees to work in.

It took UNMOVIC a few more days to sift through the reams of material necessary to discover the original source of the bag. From inventory records, it was identified as phosgene brought in during a 1996 inspection of Iraq’s al-Muthana chemical plant. Yesterday, American authorities were notified, and by late afternoon the suspicious bag was transferred by the FBI to a federal laboratory in Edgewood, Md., where it was to be tested and destroyed.

Since June — when the U.N. Security Council finally decided to wind down the mandate of the out-of-work weapons inspection team, which left Iraq on the eve of the war in March 2003 after searching for weapons of mass destruction since the 1991 Gulf War — UNMOVIC scientists have been archiving their old inspections data.

Yesterday, during a brief visit by The New York Sun to UNMOVIC’s sixth-floor offices on 48th Street, cardboard boxes filled with documents were piled up against the walls, as New York police officials and UNMOVIC employees milled around.

“We have 1,400 linear feet of files,” Mr. Buchanan said, adding that the employee who found the suspicious substance, whose name he withheld, had been transferring files to boxes meant to be sent to the archives.

During the decade and a half the United Nations was charged with weapons inspections in Iraq, staffers at UNMOVIC and its predecessor inspection team, UNSCOM, were supposed to transfer any hazardous materials they found to national laboratories like the one in Edgewood. Unlike the U.N. offices in New York, such labs have the facilities to examine and destroy the material safely. In this case, according to Mr. Buchanan, a UNSCOM agent apparently failed to make such a transfer in 1996 and the small quantity of phosgene somehow remained in a U.N. box for 11 years. “It could have been inert,” Mr. Buchanan said.

The U.N. yesterday promised an investigation.