The container was within a storage igloo at Blue Grass Army Depot that is monitored every day to ensure that its seals are effective, the Lexington Herald-Leader reported. The leak was discovered July 11.

Employees found that only vapor had leaked and no liquid had escaped the container, the Army said. They covered the leaking plug with a sealant and also covered the other plugs on the container.

Sarin, classified by the United Nations as a weapon of mass destruction, was first produced by the German government during World War II and later by both the United States and Soviet Union. Its most notorious use was by the cult Aun Shinrikyo, which killed 12 people in a 1995 attack in the Tokyo subway.

The Army said that monitoring will continue until the sarin is neutralized or destroyed.