Were there warning signs before the Christchurch terror attack?

March 15, 2019 was one of New Zealand's darkest days. We now know that the Australian terrorist who shot dead 51 people during Friday prayers at two Christchurch mosques had meticulously planned his attack. But New Zealand's royal commission into the attack which reported in December 2020 concluded "there was no plausible way" he could have been detected in advance "except by chance". The authors of a new book take issue with that statement, arguing the terrorist's online activity gave clear indications of his intentions.