The eight people still missing after the eruption of a volcano off New Zealand that killed five people are also presumed dead, and authorities are opening a criminal investigation, police said Tuesday.

At least one of the eight missing people was among nine Americans who were on the island at the time of the eruption, Prime MInister Jacinda Ardern said Tuesday.

Forty-seven people were on Whakaari White Island when its cone volcano erupted on Monday, John Tims, deputy commissioner of the national police, said at a news conference Tuesday afternoon. In addition to the five who were previously declared to have died, 31 others remain in hospitals and eight others are still unaccounted for, he said.

“Everyone that could be taken from the island yesterday were rescued at the time of the evacuation. A number of flights were carried out throughout the day, and no signs of life were seen,” Tims said, adding: “There are eight people missing and presumed deceased.”

Authorities said 25 of the 31 injured people were in burn units and that it was possible that not all of them would survive.

In addition to the nine Americans, Tims said the others who were on the island were 24 Australians, five New Zealanders, four Germans, two Chinese, two Britons and a Malaysian.

Without providing details, Tims said police were launching a criminal investigation. Asked whether the investigation would focus on tour companies that took some of the people to the island, he said: “It’s early days yet, so we’re just going to have to work through the evidence, talk to people and conduct the investigation.”

New Zealand’s workplace safety regulator said it was also conducting a work and safety investigation.

Tourists from at least one tour cruise ship were on the island in the Bay of Plenty, about 30 miles off the northeast New Zealand coast, when the country’s most active cone volcano, erupted at 2:11 p.m. Monday (8:11 p.m. Sunday ET), according to GeoNet, the government earthquake agency. Authorities said there were two eruptions that occurred in quick succession.

Jonathon Fishman, a spokesman for Royal Caribbean Cruises, confirmed that multiple guests aboard its ship Ovation of the Seas were touring the island, which in quieter times is a tourist attraction popular with birdwatchers.

Raymond Cas, an emeritus professor of geosciences at Monash University in Melbourne, Australia, told Radio New Zealand that he believed tourists shouldn’t have been on the island.

“The fact that the island is uninhabited — it has no emergency services on the island, which is very remote — is the first major concern,” Cas said Tuesday night.

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Multiple vents in the crater emit sulfur, a noxious chemical that can cause asphyxiation, Cas said, and crater lakes filled with boiling water and mud can overflow, creating a mortal hazard.

“When there are so many hazards in such a confined, small crater — you’re actually in the crater of a major volcano — that is not a good place to have a lot of people,” he said.