Two teenage brothers originally from Chicago have been identified as among those killed in Mondays volcanic eruption off the coast of New Zealand, as the official death toll from the natural disaster was raised from six to eight.

Matthew Hollander, 13, and Berend Hollander, 16, who had moved from the U.S. to Sydney with their parents, died after being hospitalized for injuries sustained while touring White Island, the Chicago Tribune reported , citing an announcement from their school.

Their mother, Barbara Hollander, 50, who was born in Chicago, and their father, Martin Hollander, 48, who was born in Australia, remained missing as of Thursday morning, local time.

The volcano erupted just after 2 p.m. Monday, covering the area with ash and steam as several tourist groups were visiting. Authorities said there were at least 47 people on the island when the blast occurred.