Their testimony was part of the government's case against Jack Reimer of Carmel, New York, who is accused of helping the Nazis round up and exterminate Jews in Poland during the Second World War.
"They were cruel beyond imagination," Carl Langner, 72, testified.
Federal prosecutors allege that Mr Reimer took part in the Holocaust but lied about his activities so he could emigrate to the US. They are asking Judge Lawrence McKenna to revoke his citizenship.
The survivors, who spoke at the non-jury trial in Manhattan federal court, had been imprisoned at camps where Mr Reimer allegedly worked. They did not identify Mr Reimer but painted graphic pictures oftorture by the guards.Reuse content