In his contemporary manuscript, Marcel Nadjari, a prisoner of the Jewish Sonderkommando in Auschwitz-Birkenau, provided the following description of crematorium 2/3 (he was usually employed at the latter):
"It is a big building with a wide chimney and 15 ovens. Below a garden there are two big endless rooms in the basement. The one serves for undressing and the other is the death chamber, where the people enter naked and after filled with about 3000 people, it is closed and they gas them, where they gave up the ghost after 6 to 7 minutes of martyrdom."
(Contemporary Handwritten Letter of Auschwitz Sonderkommando Prisoner Marcel Nadjari Deciphered, my emphasis)