Four hundred missing pages of the diary of prominent Nazi ideologue Alfred Rosenberg have been found by the US government, according to a preliminary assessment prepared by the United States Holocaust Memorial Museum.

The diary is expected to shed new light on meetings Rosenberg held with Adolf Hitler, Hermann Goering, and Heinrich Himmler, Reuters reported. It also includes new information on internal disputes within the Nazi high command, especially tensions in the wake of deputy fuhrer Rudolf Hess’s 1941 solo flight to Scotland in a quixotic attempt to broker peace with the United Kingdom.

Details of German plans to exterminate Jews and other peoples in eastern Europe are also contained in the journal.

The diary contains Rosenberg’s handwritten entries from spring 1936 to winter 1944. According to the museum’s report, some of the recollections are written on the back of official Nazi stationery.

Rosenberg was a close associate of Hitler’s, and played a prominent role in planning the Holocaust. In addition to heading the Nazi party’s foreign affairs department and editing the party newspaper, Rosenberg served as Reich minister for the occupied eastern territories, giving him responsibility for the extermination of Jews in the Soviet Union. He also established the Einsatzstab Rosenberg, a body dedicated to looting cultural treasures from across the Continent to bring to Germany. Much of the art came from Jewish collectors.

After standing trial before the International Military Tribunal at Nuremberg in 1945 for crimes against humanity, Rosenberg was hanged on October 16, 1946, along with other senior Nazis. His diary was used as evidence by the prosecution, but mysteriously disappeared after the trial.

American officials suspected that Robert Kempner, assistant US chief counsel at Nuremberg, himself a concentration camp survivor, smuggled the diary back to the United States.

Over the ensuing years, excerpts from the diary occasionally surfaced. Entries from 1939 and 1940 were published by a German historian, and Kempner himself cited the journal in his memoir. Still, the book was not found.

After Kempner’s death in 1993, a fight over his papers broke out between his family, his secretary, and the Holocaust Memorial Museum. Kempner’s children finally agreed to release the papers to the museum, where it was discovered that thousands of documents had gone missing.

The FBI opened an investigation into the case, but filed no charges.

Reuters reports that more than 150,000 documents were recovered by the museum, a significant portion of which were found in the hands of Kempner’s former secretary, who was living with an academic named Herbert Richardson in New York. But the diary was not among the found documents.

In early 2013, the Homeland Security department and the Holocaust museum finally located the diary in Richardson’s home.

“The documentation is of considerable importance for the study of the Nazi era, including the history of the Holocaust,” the Holocaust Memorial Museum said in a statement. “A cursory content analysis indicates that the material sheds new light on a number of important issues relating to the Third Reich’s policy. The diary will be an important source of information to historians that complements, and in part contradicts, already known documentation.”

A press conference on the discovery is expected in Delaware later this week.