New Zealand Prime Minister Jacinda Ardern said on Saturday that world leaders and tech firms looking to stamp out violent extremism online would need to focus efforts on understanding social media algorithms that drive content.

Ardern was speaking at a virtual summit to mark the second anniversary of the global initiative to end online hate, called the Christchurch Call, launched by Ardern and French President Emmanuel Macron in 2019 after a white supremacist killed 51 people at two mosques in the New Zealand city of Christchurch while live-streaming his rampage on Facebook.

Since then more than 50 countries, international organizations and tech firms have supported the initiative including firms like Facebook, Google, Twitter and Microsoft.