Area residents were never at risk, officials said, because the toxic vapor never escaped from the sealed container, where about 2,500 rockets are stored and awaiting destruction.

The hands-on inspection of the rockets was delayed until officials determined whether sensors could locate the source of the leak, said Richard Sloan, a spokesman for the Army's Blue Grass Chemical Activity.

But after air in the chamber was filtered out, monitors did not pick up new leaks of the colorless chemical agent, Sloan said.

On Monday, workers wearing protective clothing and carrying antidote shots plan to enter the chamber and break down stacks of rockets to check them. Seeping rockets will be sealed in a leak-proof container.

"This may be a long process because of the low amount of vapor so far detected and the large amount of rockets in the igloo," Lt. Col. George Shuplinkov, commander of the chemical activity, said in a statement.

The rockets are made of different types of metals, so officials say slight chemical reactions between them could cause small amounts of pressure to build in the canisters and the tubes that hold them. The leaked vapor may have escaped in a puff or hiccup, Sloan said.

In the past two decades, 71 rockets stored at Blue Grass have leaked, Sloan said.

Sarin gas is one of the world's most toxic and rapidly acting chemical warfare agents. Originally developed as a pesticide in Germany in 1938, sarin is a clear, colorless liquid that has no odor in its pure form. It can evaporate into a vapor and spread into an environment.

In 1995, terrorists in Japan released sarin gas into the city's subways, killing 12 people and sending thousands more to hospitals.

This week's incident at Blue Grass was the first sarin leak at the depot since 2000. The last time a chemical weapon leaked was in 2003, when mustard vapor seeped from an old projectile.