Decades after his family was ousted from their attic hiding space, Otto Frank began corresponding with a pen pal named Ryan Cooper. Throughout the 1970s, Frank and Cooper exchanged letters, with Frank offering perspectives on his time in seclusion and captivity during World War II. His daughter Anne’s famous diary was written while the family was hiding from German forces in Amsterdam.

Now, the United States Holocaust Memorial Museum is making those letters available digitally for the first time to commemorate what would have been Anne’s 90th birthday on June 12.

Cooper, an artist in California who was then in his 20s, struck up a pen pal relationship with Frank. In addition to garnering advice on a variety of topics, Cooper was able to learn more about the young woman whose Diary of Anne Frank went on to become one of the best-known chronicles of the war and who tragically died of typhus while being held in a concentration camp in 1945. The letters also reveal more about Otto Frank, who appeared determined to keep the memory of his daughter alive even as his own health began to deteriorate. Frank died in 1980 at the age of 91 as the family's only survivor of the war.

Cooper amassed more than 80 letters in total, including some from Miep Gies, who protected Anne’s writings until the war ended. The museum is expected to make all of it accessible online in the near future.

[h/t Smithsonian]