The message he sent right before the attack will likely not have been his first communication about it. There's been a pattern of Isis operatives abroad guiding and supporting terrorists in detail via end-to-end encrypted messaging for weeks or months, right up until the moment they attack: https://www.nytimes.com/2017/02/04/world/asia/isis-messaging... Presumably the British police are assuming that this is like those previous attacks, but they haven't managed to obtain the actual message contents after the fact this time around for some reason.