Historian Peter Longerich states that Hitler "avoided giving a clear written order to exterminate Jewish civilians." Wide protest was evoked when Hitler's authorisation of the T4 program became public knowledge in Germany, and he was forced to put a halt to it as a result (nonetheless it continued discreetly). This made Hitler realise that such undertakings must be done secretly in order to avoid criticism. Critics also point out that if Hitler did sign such an order in the first place, it would have been one of the first documents to be destroyed."