TOKYO, Japan (AP) — Rescuers said that 10 people who were retrieved Sunday from the frigid sea and the rocky coast of a northern Japanese national park had died, a day after a tour boat with 26 aboard apparently sank in rough waters, triggering questions why it was allowed to sail.

The search for the others is still ongoing after the boat sent a distress call on Saturday afternoon saying it was sinking. The location, near the Kashuni Waterfall, is known as a difficult place to maneuver boats because of its rocky coastline and strong tide.

There were two crew and 24 passengers, including two children, on the the 19-ton Kazu 1 when it ran into troubles while travelling off the western coast of the Shiretoko Peninsula. The coast guard said the 10 victims — seven men and three women — were adults.

The Transport Ministry launched an investigation into the boat’s operator, which had two accidents last year. The ministry said it was looking into safety standards and the decision to conduct the tour despite rough weather on Saturday.

“We will thoroughly investigate what caused this situation and what kind of safety oversight was involved to allow the tour in order to prevent another accident,” Transport Minister Tetsuo Saito, who visited the area Sunday, told reporters.

Following an intensive search involving six patrol boats, several aircraft and divers that went through the night, rescuers on early Sunday found four people near the tip of Shiretoko Peninsula and later six more in the same area, about 14 kilometers (8.7 miles) north from where the boat sent a distress call. Some of them were plucked from the sea, while others were washed onto the rocky coast.

Prime Minister Fumio Kishida cut short his attendance at a two-day summit in Kumamoto in southern Japan and returned to Tokyo. He told reporters early Sunday that he instructed officials “to do everything they can for the rescue.”

The cause of the accident is under investigation, but officials and experts suspect a safety negligence.