Local authorities resumed their search Tuesday for a 37-year-old man and his 6-year-old son who went missing while hiking on a mountain in Niigata Prefecture two days ago.

Koya Shibuya and his son Sora have not been heard from since telling their family by mobile phone on Sunday morning that they were beginning their descent of the mountain in the central Japan prefecture, now thought to be Mt. Matsudaira. Family members reported them missing later that day, police said.

According to the police, Shibuya called his father on Saturday evening to say that he and his son would sleep on the mountain as they were lost.

Police and rescue workers started the search on Monday in rainy, low-visibility conditions but did not locate the pair before suspending operations in the evening. A police helicopter joined the search effort Tuesday morning.

The police initially believed that the father and son planned to tackle two mountains, both about 500 meters high. But a note was later found at the foot of Mt. Matsudaira indicating they intended to head to the top of the 954-meter mountain.

Two casually-outfitted hikers were reportedly seen on a Mt. Matsudaira trail, prompting police and rescue workers to extend the search area.

Climbing Mt. Matsudaira is not recommended for beginners as its trails are steep, according to a local government.