Lighting sparked a forest fire in mountainous western China last month that killed 27 firefighters and four local helpers in one of the country’s deadliest days for first responders, investigators said.

Acting on accounts of eyewitnesses in Sichuan province’s Muli county, investigators located an 18-meter (59-foot) pine tree split by lightning that they identified as the fire’s origin point. They said that after igniting the tree, the fire spread to the thick layer of decomposed plant material on the ground known as humus lying in a remote area at an altitude of around 3,800 meters (12,500 feet.)