Archival silences in regard to Native Americans could fill more than one book, no doubt. But for now, here’s an example:
In a tiny office at North Dakota State University in Fargo, Clifford Canku has spent 10 years poring over the faint handwriting with a magnifying glass.
“One letter would take about a week,” said Canku, a Dakota elder who teaches Dakota language at North Dakota State. Canku is one of three lead translators on the project, which has unearthed never-before revealed details of a turbulent episode in Minnesota history.
The letters are from imprisoned Dakota men, and are important and revealing on many levels. Here’s the link to the story: http://www.mprnews.org/story/2011/01/19/dakota-tribe-letters
The written Dakota language was created by a Presbyterian missionary, Stephen Riggs. When the prisoners wrote to him, he would share the letters with families. The letters, along with other documents, were stored in a box at the Minnesota Historical Society for decades.
So this is a case when there is documentation, but it was inaccessible due to the rarity of the language (I assume) and also the difficulty of reading the fading ink of the letters.