<p>Who could do such a thing? It’s the question Australians are asking after Sunday’s mass shooting at Bondi Beach.</p><p>How did they radicalise? Who did they associate with? And what drove them to plan and execute the worst terror attack in Australia’s history?</p><p>Today, investigative reporter Sean Rubinsztein-Dunlop on what we know about the shooters and the unresolved questions for our security services.</p><p>Featured: </p><p>Sean Rubinsztein-Dunlop, ABC investigative reporter</p>