Heinrich Matthes testifies about Treblinka
During the entire time I was in Treblinka, I served in the upper camp.
The upper camp was that part of Treblinka with the gas chambers,
where the Jews were killed and their corpses laid in large pits and
later burned.

About fourteen Germans carried out services in the upper camp. There
were two Ukrainians permanently in the upper camp. One of them was
called Nikolai, the other was a short man, I don't remember his name...
These two Ukrainians who lived in the upper camp served in the gas
chambers. They also took care of the engine room when Fritz Schmidt
was absent. Usually this Schmidt was in charge of the engine room. In
my opinion, as a civilian he was either a mechanic or a driver...

All together, six gas chambers were active. According to my estimate,
about 300 people could enter each gas chamber. The people went into
the gas chamber without resistance. Those who were at the end, the
Ukrainian guards had to push inside. I personally saw how the
Ukrainians pushed the people with their rifle butts...

The gas chambers were closed for about thirty minutes. Then Schmidt
stopped the gassing, and the two Ukrainians who were in the engine
room opened the gas chambers from the other side.

BELZEC, SOBIBOR, TREBLINKA - the Operation Reinhard Death Camps, Yitzhak Arad, p. 121