The Nazis murdered thousands of disabled people they thought unfit to live, deeming it “euthanasia”. The hospital believes many died there as part of the programme.
A planned construction project has been halted to allow a full investigation.
Some 30,000 people were killed at one psychiatric hospital alone – Schloss Hartheim, near Linz in upper Austria.
Tilak, the company responsible for the Hall hospital, said the graves contained the remains of people buried between 1942 and 1945.
There were, it added, “suspicions that the dead [were] at least partially victims” of the programme of murders the Nazis referred to as “euthanasia”.