Official notice from the Red Cross from 1954 that Anne Frank died in the Bergen-Belsen concentration camp, somewhere between 1 and 31 March, 1945. Otto Frank had already heard of both his daughters' deaths in the summer of 1945, from a nurse who was in the same camp as Anne en Margot Frank. It became clear later that the date of death must have been between late February and early March, 1945. Anne died a few days after her sister Margot.
Official Red Cross declaration of Anne Frank's death, May 5, 1954.