Beginning on 17 January, some 58,000 Auschwitz detainees (about two-thirds Jews)—over 20,000 from Auschwitz I and II and over 30,000 from the subcamps—were evacuated under guard, at first heading west on foot, then by open-topped freight trains, to concentration camps in Germany and Austria: Bergen-Belsen, Buchenwald, Dachau, Flossenburg, Gross-Rosen, Mauthausen, Dora-Mittelbau, Ravensbruck, and Sachsenhausen. Fewer than 9,000 remained in the camps, deemed too sick to move. During the marches, the SS shot or otherwise dispatched anyone unable to continue; "execution details" followed the marchers, killing prisoners who lagged behind. Peter Longerich estimated that a quarter of the detainees were thus killed. By December 1944 some 15,000 Jewish prisoners had made it from Auschwitz to Bergen-Belsen, where they were liberated by the British on 15 April 1945.