I wrote about the liberation of the Mauthausen camp on this previous blog post: https://furtherglory.wordpress.com/2010/05/07/the-liberation-of-mauthausen-may-5-1945/
Mauthausen was the only Class III camp in the concentration camp system. Prisoners who were sent there were classified as “Return Unwanted” meaning that they had almost no chance of being released.
There was one American who was a prisoner at Mauthausen: Lt. Jack Taylor. You can read about him on this previous blog post: https://furtherglory.wordpress.com/2011/05/06/lt-cmdr-jack-h-taylor-americas-first-navy-seal/
The Mauthausen camp is unique because of these three things:
1. The Mauthausen camp is very beautiful; the town of Mauthausen is even more beautiful.
2. More lies have been told about this camp than about any other Nazi concentration camp. The biggest lie ever told is the lie about the Mauthausen gas chamber, which was a multi-purpose room where prisoners could take a shower, or be killed by gas put into the room through an apparatus that was removed just before the camp was liberated.
3. There are more memorials to the former prisoners of this camp than there are in any of the other Concentration Camp Memorial Sites.

The Memorial to the Jews, who were killed at Mauthausen, is in the most prominent spot (Click on photo to enlarge)