National Archives and Records Administration

The National Archives and Records Administration (NARA) is an independent agency of the United States government charged with preserving and documenting government and historical records and with increasing public access to those documents, which comprise the National Archives. NARA is officially responsible for maintaining and publishing the legally authentic and authoritative copies of acts of Congress, presidential proclamations and executive orders, and federal regulations.

The chief administrator of NARA is the Archivist of the United States, who not only maintains the official documentation of the passage of amendments to the U.S. Constitution by state legislatures, but has the authority to declare when the constitutional threshold for passage has been reached, and therefore when an act has become an amendment.

More information on the Wikipedia page.

The Website is.