The Vatican said Friday it will release from its secret archives years of files on Pope Pius XI, whose pontificate spanned most of the period between the world wars.

The files cover the period from Feb. 6, 1922, to Feb. 10, 1939. Researchers will be allowed to access the files beginning Sept. 18.

In 2003, the Vatican began making available to scholars millions of Vatican documents from the years leading up to World War II, making them available ahead of schedule at the request of Pope John Paul II.

For years, the Vatican has struggled to defend Pius' successor — the wartime Pope Pius XII — against claims he did not do enough to save Jews from the Holocaust.

The material to be released may include an encyclical that Pius XI commissioned to denounce racism and the violent nationalism of Germany.

The encyclical was titled "Humani Generis Unitatas," or "The Unity of the Human Race," but Pius died before releasing it. It was never made public.