The Navajo Nation is working to acquire one of the three copies of a treaty between the tribe and the United States.

The treaty, which dates back to 1868, was signed on June 1 and acknowledges tribal sovereignty for the Navajos after they were forcibly removed and confined in Eastern New Mexico. Clare Weaver, a descendant of an Indian peace commissioner Samuel F. Tappan discovered the version of the treaty. Tappan was involved in the treaty’s negotiation process.

Weaver decided to donate the treaty copy to the tribe after discovering it in her home’s attic in Manchester, Massachusetts. The version signed by federal officials and Navajo leaders is kept at the National Archives and Records Administration in Washington D.C.

A third copy was given to a Navajo leader and its location is unknown. The Albuquerque Journal reports the letter would be housed at the Navajo Nation Museum in Window Rock, Arizona.