The Declaration of Independence, Constitution and Bill of Rights have been stored in a special 50-ton steel and concrete vault since they were transferred to the National Archives Building from the Library of Congress in 1952.

Each morning, the helium-filled cases that hold the three documents are raised from the vault by a scissors-action jack to the special shrine where they are displayed, and each night they are lowered back into the vault.

It was the protective glass of the display case, and not the hermetically sealed document cases, that was damaged in yesterday's attack.

The helium-filled cases, in which the so-called freedom documents are kept, are maintained at a temperature of 70 degrees and constant humidity of 25 to 30 percent. The cases also protect the documents from ultraviolet light, which is especially damaging to the fragile parchment used for the documents.

Archives spokeswoman Jill Brett said the vault in which the documents are stored is fireproof and waterproof. When it opened in 1952 in the midst of the Cold War, the vault was considered atomic bomb-proof. At all times when the documents are on exhibition, they are protected by two armed guards, who can activate the vault mechanism at any time.

The exhibition hall in which the historic documents are displayed was built specifically to display them, although the three documents were not moved to the Archives until nearly 17 years after the building was completed.

The documents are displayed on a platform in a circular marble hall. The Declaration of Independence stands upright, and the first and last pages of the Constitution and the Bill of Rights are laid out before it. The other two pages of the Constitution are also kept in helium-filled, sealed cases, but they are rarely displayed.