The accused mass murderer who shot 50 people to death in two New Zealand mosques acted alone, but investigators haven’t ruled out that he had support from an accomplice, the country’s police chief said Monday.

“We believe absolutely there was only one attacker responsible for this,” New Zealand Police Commissioner Mike Bush said in a press conference on Monday.

But, Bush added, police are looking into whether he had support from other people. It is “a very, very important part of our investigation,” he said.

Australian white supremacist Brenton Tarrant was arrested after the two shootings on Friday and charged with a single count of murder, authorities said. More charges are expected to be filed against him.

Tarrant, armed with five guns, allegedly stormed two mosques in the city of Christchurch and slaughtered 50 people who were there for prayers.

Tarrant livestreamed his attack on Facebook and left behind a hate-filled manifesto.

In response to the attack, which was the worst terror attack in the country’s recent history, Prime Minister Jacinda Ardern has said the nation’s gun laws will be amended.