Japanese police raided the offices of a tour organizer Saturday on suspicion of negligence after eight senior citizens died of apparent hypothermia in cold, wet weather during a climbing tour of Japan's northern mountains, officials said.

The hikers, most of them in their 60s, were found dead Friday. They were climbing Mount Tomuraushi on Hokkaido, Japan's main northern island, police official Tsuyoshi Matsuya said.

The members of an 18-member tour organized by Amuse-Travel Co. Ltd. were found at separate locations on the mountain, a sign that they were not well escorted, the police official said.

Another senior citizen died on the mountain in the same period while hiking alone, and a 10th died on another mountain on Hokkaido. Police and media reports said bad weather affected their descent and all 10 died sometime between Thursday and Friday.

The striking number of deaths exposed the risks of mountain climbing as a trend among Japanese senior citizens as a way to stay healthy. Yoshiaki Takeda, a local official near the 7,024-foot Mount Tomuraushi, said it was highly unusual for deadly accidents to occur on the mountain, which has several shelters.

The climbers had thin rain jackets that apparently were insufficient for the strong winds, chilly mountain air and rain, Matsuya said. The temperature in the area was about 46 degrees Fahrenheit (8 degrees Celsius), a few degrees lower than usual.

Investigators raided Amuse-Travel Co.'s headquarters in Tokyo and its regional office on Hokkaido on Saturday, and are questioning company officials, including its president, though none has been arrested, Matsuya said.

Tour agency president Seiichi Matsushita apologized to the victims' families Friday, but defended his company and its tour guides, saying they took sufficient safety measures.

Public broadcaster NHK said two of the three guides had never climbed the mountain. Police declined to confirm the report.

More than 60 percent of the 1,993 people who were involved in mountain accidents in fiscal 2008, including 281 deaths, were climbers aged 55 or older, according to the National Police Agency.