Who prevents the evacuation of the last 20,000 prisoners at Buchenwald?

This question refers to the events of chapter 9. In the text, Elie and the other prisoners learn that Buchenwald is to be evacuated and the camp liquidated in stages in April. Elie is among the twenty thousand remaining prisoners in the camp, including several hundred children.
The last twenty thousand were to be evacuated into the evening on April 10, but as Elie is waiting in the assembly square before the gates, the sirens in the camp suddenly go off.
The prisoners are then instructed to return to their barracks to await further instructions. By the time the sirens have stopped, it is too late in the evening to begin evacuations. The next day, Buchenwald's prisoners' resistance group makes a move against the remaining SS in charge of the camp, who abandon the camp rather than continue fighting to maintain control. Elie explains that at six o'clock that evening, the first American tanks arrived at the camp, thereby signifying its liberation.
Although the text does not explicitly state who is responsible for these sirens, the reader can infer that it is the approaching American forces that caused the Germans in control of the sirens to sound the alarms to prevent evacuation.