The Simon Wiesenthal Center has an online list of questions about the Holocaust; you can read the answers to the questions here.
Here is the answer to Question #12, quoted from the website:
Did the Nazis plan to murder the Jews from the beginning of their regime?
Answer: This question is one of the most difficult to answer. While Hitler made several references to killing Jews, both in his early writings (Mein Kampf) and in various speeches during the 1930s, it is fairly certain that the Nazis had no operative plan for the systematic annihilation of the Jews before 1941. The decision on the systematic murder of the Jews was apparently made in the late winter or the early spring of 1941 in conjunction with the decision to invade the Soviet Union.
The decision “was apparently made?” How do we know that the decision was made at all? Apparently, the decision was not put on paper. Note that the Simon Wiesenthal Center’s answer to this question does not explicitly say that Hitler made this decision nor that Hitler was the one who gave the order. Apparently someone read Hitler’s mind and no order was even necessary. (more…)