Mr. Shapiro said he had no estimate as to how many of the roughly 20,000 shoulder-fired antiaircraft missiles that had been in Libya were unaccounted for since the fall of Colonel Qaddafi, but added, “In the wrong hands these systems could pose a potential threat to civil aviation.”

The State Department so far has sent 14 unarmed civilian contractors, many with military experience, to be part of teams led by Libya’s Transitional National Council, according to David I. McKeeby, a department spokesman. Mr. McKeeby said that an additional two to three dozen contractors would join the effort over the coming weeks.

The teams have surveyed and secured 20 of the former government’s 36 known ammunition depots, encompassing several hundred bunkers at each site, and have destroyed or disabled hundreds of the shoulder-fired missiles, he said. The deployment of the American contractors was reported on Friday by The Washington Post.

Though Western officials said they had asked rebel officials to secure Libya’s vast arsenal, there was little evidence on the ground of that effort. For months during the conflict, the capture of towns and cities was quickly followed by a familiar routine, in which rebel fighters — along with most anyone who showed up — converged on abandoned arsenals and emptied their stores.

After the initial looting, many depots remained unguarded or tended only occasionally for months.

Some depots appeared to have been bombed by NATO, but in many cases the airstrikes failed to destroy all of the weapons. In June, when rebel fighters reached an enormous depot in the mountains, they found many bunkers destroyed but also several full of weapons, including SA-7 components and antitank weapons. Several days later, the rebels said NATO planes had returned to bomb the depot again.