On the Wikipedia page for the Black Hole Information paradox, it states that
More precisely, if there is an entangled pure state, and one part of the entangled system is thrown into the black hole while keeping the other part outside, the result is a mixed state after the partial trace is taken into the interior of the black hole. But since everything within the interior of the black hole will hit the singularity within a finite time, the part which is traced over partially might disappear completely from the physical system.
No citation is given, and I have been trying to find another resource that gives me a similar description of the loss of information in a black hole, but I've found nothing. If someone could give me a solid reference for this idea (which could be understood by someone with a Thorne, Misner & Wheeler level of understanding of GR), it would be greatly appreciated. If the Penrose diagram on that same page described an eternal black hole (i.e., no Hawking radiation), would information still be lost?