The forest is seen after the blaze. [Photo/Chinanews.com]

At about 5 pm Saturday, a small spot of the burned forest caught fire, burning rotten wooden poles which subsequently rolled down a cliff, igniting trees that had not been totally burned.

The blaze then spread outside the previously burned area, according to the Liangshan office of the forestry and grassland fire prevention headquarters.

A lightning strike has been confirmed as the cause of the blaze, which started on March 30.

Li Guangjun, head of the Liangshan forestry police said his bureau started an on-the-spot investigation on Tuesday and found an 80-year-old pine tree on the ridge of the mountains in the village of Li'er, Muli county that was believed to be the starting point of the blaze.

It had obvious traces of a lightning strike and fire, Li said.