Official notice from the Red Cross from 1954 that Margot 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. Margot died a few days before her sister Anne.
Official Red Cross declaration of Margot Frank's death, May 5,1954.