Nine seniors died while climbing Mount Tomuraushi on Hokkaido, Japan's main northern island, said police spokesman Masafumi Yamasaki. Eight were part of an 18-member group tour organized by Amuse-Travel Co. Ltd., while the other was climbing alone.
A 10th elderly person died on another mountain on Hokkaido, he said.
Another police official, Tsuyoshi Matsuya, said investigators believe almost all the victims died of hypothermia.
"Most of the climbers were senior citizens," Matsuya said. "We are looking into a possibility of professional negligence on the part of organizers."
Investigators believe the elderly climbers, caught in strong winds and worn out, quickly deteriorated as chilly mountain air and rain sent their body temperatures dangerously low.
Matsuya said the climbers had thin rain jackets that apparently were insufficient for the harsh weather.
The temperature in the area was about 46 degrees Fahrenheit (8 degrees Celsius), a few degrees lower than usual.
Tour agency president Seiichi Matsushita apologized to the victims' families, offering to "do everything our company can do," but he defended his company and tour guides, saying they took sufficient safety measures.
"Nature and environment often affect mountain climbing, and we have repeatedly reminded our guides not to force their way when there is a slightest safety concern," Matsushita told a televised news conference late Friday. "I assume that the guides judged that the weather wasn't so bad when they decided to go on."
Yoshiaki Takeda, a local official near Mount Tomuraushi, said it was highly unusual for deadly accidents to occur on the mountain, which has several shelters.
Tadao Kanzaki, vice president of the Japanese Alpine Club, said elderly climbers tend to overestimate their ability and often make plans that "they can no longer keep up with."
Police earlier said that one person was missing on the Tomuraushi mountain, but Yamasaki said that climber was rescued.