Work has begun on a memorial in Germany for the 300,000 people murdered by the Nazis for having mental and physical disabilities or chronic illnesses.
A 30m (100ft) long glass wall is being built in the centre of Berlin, near the former site of the Nazi-era office that organised the "euthanasia" programme.
In 1939, Adolf Hitler told officials that people "considered incurable" should be "granted a mercy killing".
The programme ended officially in 1941, but continued covertly until 1945.
At first, personnel killed people by starvation and lethal injection. They later used gas chambers at killing centres in Germany and Austria.
The programme, also known as Action T4, is considered a precursor to the Holocaust, during which millions of Jews were killed.
On Monday, Germany's State Minister for Culture, Bernd Neumann, said educating people about the Nazis' crimes and honouring their victims remained an obligation for the country.
In recent years, memorials have been constructed in Berlin for Jewish, Roma (Gypsy) and homosexual victims of the Nazis.
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