Police are trying to suppress a witness account that he said this. Why? The prevailing story is that he was mentally ill. He may indeed be. But being an Islamic jihad murderer and mentally ill are not mutually exclusive propositions. Or he may be, in his mental illness, imitating the behavior of those whom he sees as strong and fearsome. In any case, the police attempt to suppress evidence is troubling.
You can hear “Allahu akbar” at :34. in the video below.