Who could do such a thing? It’s the question Australians are asking after Sunday’s mass shooting at Bondi Beach.

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?

Today, investigative reporter Sean Rubinsztein-Dunlop on what we know about the shooters and the unresolved questions for our security services.

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Sean Rubinsztein-Dunlop, ABC investigative reporter