WHAKATANE, New Zealand — Above the noise of the boat engine, there was quiet, Geoff Hopkins remembered. There was no rumble, no roar to signal that the White Island volcano had awakened.

The crater he’d just circled with his 22-year-old daughter, Lilani, erupted as they sat in a catamaran several hundred meters from shore, watching with other passengers as the island disappeared under ash.

“A rolling rumbling mass of ash tumbled over the cliff face, in all directions, and it completely engulfed the island,” said Mr. Hopkins, a 50-year-old pastor. “It cut out the sun, it went dark. You couldn’t see that there was an island there. It was completely covered in ash."

That Mr. Hopkins, his daughter and dozens of others were allowed to go near the island — let alone scale the crater at its center — when geologists had repeatedly warned of increased volcanic activity is now the subject of an investigation, with the death toll from the eruption Monday having risen to six. As of Tuesday afternoon, eight others were also believed to have died, with emergency workers still unable to reach the island to retrieve them.

And the question heard over and over in the long hours since, heard as the injured were carried to the docks, is: “Why?”