Who were the Muselmanner?

From the text:

"......the most tragic of all prisoners were the “Muselmanner,” as they were called. Wrapped in their torn blankets, they would sit or lie on the ground, staring vacantly into space, unaware of who or where they were, strangers to their surroundings. They no longer felt pain, hunger, thirst. They feared nothing. They felt nothing. They were dead and did not know it."