"The frank answer is we don't know and probably never will," said Andrew Shapiro, an assistant secretary of State.

Inspectors have accounted for about 5,000 of the portable missiles and components. Gadhafi's regime stockpiled about 20,000 portable missiles during his four decades in power.

It is not clear how many remained at the time Tripoli fell to anti-Gadhafi rebels. Many were probably destroyed by NATO airstrikes, and others remain in the hands of militias who fought Gadhafi's regime.

That makes it difficult to estimate how many remain at large. "I think it's potentially thousands," said Rachel Stohl, an analyst at the Stimson Center, a think tank. "Nobody knows."

The missiles, called man-portable air defense systems, are ideal for terrorists because they are easily concealed and can hit commercial airliners.

The portable missiles have been used in attacks on 40 aircraft, causing 28 crashes and more than 800 deaths since 1975, according to the State Department.

In Libya, many of the missiles remain in the possession of militias, which raided munition storage facilities during the rebellion and have yet to come under the control of the new government there.

The Libyan government is trying to negotiate with the militias in an effort to get them to disarm. The government does not have an army capable of forcing the militias to disband, said Ronald Bruce St John, who has written a number of books about Libya.

Sen. Susan Collins of Maine, the ranking Republican on the Homeland Security Committee, said she is concerned that the Obama administration was late in trying to get an accounting of the missiles.

Shapiro said the United States took "immediate steps" to secure weapons. The United States launched an effort to try to account for the missiles in April, before the collapse of Gadhafi's regime, and has committed $40 million in an effort to secure weapon stockpiles.

Early in the effort, the U.S. government worked through aid groups operating in Libya. Since then it has placed U.S. contractors there who are working closely with the Libyan government.