"Today the cabinet agreed the inquiry will be a royal commission… The inquiry will look at what could have or should have been done to prevent the attack," she told reporters.
Ardern added the actions of government agencies prior to the massacre would be studied, including whether the country’s intelligence community concentrated its resources appropriately.
"It is important that no stone is left unturned to get to the bottom of how this act of terrorism occurred and what, if any, opportunities we had to stop it," she said.
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An Australian man opened fire inside two mosques on March 15, killing 50 people. New Zealand's government banned military-style semi-automatic rifles shortly afterwards to make the country a safer place.