The city's fire department received an emergency call at around 6:45 a.m. saying the girl, Miki Ito, and her mother Mayumi, 45, were found collapsed at a mountain near the Sukayu hot springs resort in the city.
The girl was pronounced dead at the site by rescue workers who rushed to the scene, while the mother was taken to a nearby hospital with two of her relatives, who became sick, according to rescue workers. There is no threat to the lives of Mayumi and the two others, they added.
The police said the girl and three others, who were at the mountain picking edible wild plants, may have inhaled toxic gas that might have been generated in the area.
The mother and daughter were staying at the hot spring resort with seven people who are their relatives since Saturday evening, the police said, adding that seven of the nine-member group went into the mountain to collect wild plants on early Sunday morning.
Police are searching the mountains to see if there are more people in the area, they said.
The Hakkoda Mountains is a volcanic range that lies to the south of Aomori City.
Plausible.