Police searched a sunken tour boat off Japan's northernmost main island of Hokkaido with a drone Tuesday but found no signs of 12 people who remain missing, officials involved in the search operation said.

The authorities had struggled to search the inside of the 19-ton boat that sank more than a week ago with 26 people aboard and was found on Friday at a depth of 120 meters off the Shiretoko Peninsula.

Photo taken on April 29, 2022, by the Maritime Self-Defense Force minesweeper Izushima shows the sunken tourist boat Kazu I off the Shiretoko Peninsula in Hokkaido, northern Japan. (Photo courtesy of 1st Regional Coast Guard Headquarters)(Kyodo)

While previous attempts to search the ship with the camera-equipped device failed due to strong swells and currents, the Hokkaido police managed to send the drone into the submerged vessel a few times on Tuesday, according to Japan Coast Guard officials taking part in the operation.

Kazu I, operated by Shiretoko Yuransen and carrying 24 passengers and two crew members, went missing on April 23 after leaving port in Shari, Hokkaido, for a cruise along the peninsula designated as a World Natural Heritage site despite a bad weather warning.

Fourteen deaths have been confirmed so far. The authorities continued searching for the missing Tuesday with 12 ships and six aircraft.

Preparations are being made for a deep-diving operation to take a closer look inside the sunken boat.

The boat's operator was found to have violated safety rules, according to sources close to the investigation.

The captain of the boat routinely failed to report on the progress of each cruise to the operation manager Seiichi Katsurada, who is also the company's president, as required by company rules, the sources said.

On the day of the accident, Katsurada was not in his office and did not instruct other employees to stay in contact with the boat's skipper, they said.