How did Bruno and Shmuel die in The Boy in the Striped Pajamas by John Boyne?

Bruno is
a nine-year-old boy from whose perspective byis told. The story starts in
Berlin, but Bruno's father is a Commandant in the Nazi army and gets promoted by Hitler himself.
This promotion comes with a new assignment: the commandant of the concentration (death) camp
called Auschwitz. Bruno is naive and tells the story just as he sees and experiences
it.

Bruno is out exploring one day and discovers another little boy sitting
morosely on the other side of the fence. His name is Schmuel and he is exactly the same age as
Bruno; they even share the same birthday. Their relationship continues over time, but eventually
Bruno is going to be moving back to Berlin with his sister and mother and he wants to play at
least one time on the same side of the fence with his best (and only) friend, Schmuel.

Schmuel has been unable to find his father (we deduce that his father has been
exterminated), so Bruno agrees to come inside and help his friend look for clues. Schmuel brings
Bruno a uniform (which Bruno refers to as "striped pajamas"), and Bruno fits right in
since his head had been shaved because of lice--though he is "fatter" because he has,
of course, been eating well.

The boys discover nothing as they explore the
camp, but just as Bruno is considering sneaking back under the fence and go home, some soldiers
come and round the two boys and some others up and herd them into 

a long room that was surprisingly warm and must have been very securely built because
no rainwas getting in anywhere. In fact, it felt completely airtight.

Bruno assumes the soldiers are just being nice and trying to keep
this group from standing in the rain and catching colds. Bruno and Schmuel are glad to have each
other as they wait together in the dark, neither realizing what is about to happen to
them. 

The next line of the novel says this:

Nothing more was ever heard of Bruno after that.

Obviously what happened is that Bruno, Schmuel, and the others the soldiers rounded up
that day were gassed to death and presumably their bodies were then
burned.