Harold Denny, who visited Buchenwald as a correspondent of the New York Times, described the death house:

It is a neat brick building. […]

At the side of the death house is a stairway leading into a basement. This basement is paved with concrete sloping toward drains and equipped with hoses. Beside the stairway, a chute led down into this basement on the outside. Prisoners told me, and the equipment fitted their accounts, that it was the practice of the SS executioners to bring truckloads of Jews into the courtyard and drop them one by one down the chute. At the bottom one SS guard slipped a noose around the victim’s neck while his companion brained the victim with a club. Then they dragged his body across the floor to a large, electrically operated elevator. Calculated, mechanized murder.

When the elevator was filled, its cargo was hoisted to the floor above, where stand six neat coal-burning furnaces for cremating them. Three bodies could be put in each at one time. In these furnaces today are partly burned bodies of their last victims.

Source:

The Black Book of Polish Jewry, 1946 (English-language edition), page 256.