(COPENHAGEN, Denmark) — A cruise ship that was the focus of a daring rescue operation off Norway’s frigid North Sea coast became disabled because its engines didn’t have enough lubricating oil, the country’s top maritime official said Wednesday.

Low oil levels were the “direct cause” of the engine failure that stranded the ship during a storm Saturday, Lars Alvestad, the acting director general of the Norwegian Maritime Authority, said. Sensors detected the oil shortage and automatically shut down the Viking Sky’s engines to prevent a breakdown, he said.

The ship’s harrowing weekend ordeal injured dozens of people, including 36 who were admitted to hospitals. Four people from the ship remained hospitalized Wednesday, including one being treated in an intensive care ward in critical but stable condition, Norwegian health officials said.