The blaze started near a landfill in El Peral rural estate in Valparaiso, located along the La Polvora road, Valparaiso's regional prosecutor Eduardo Fernandez was quoted by national daily La Tercera as saying.
The authorities reached this conclusion based on accounts of eyewitnesses, mainly a caretaker working for the National Forest Corporation (Conaf), Fernandez said.
The blaze then spread toward the hills, destroying about 2,000 homes and forcing thousands of residents to evacuate.
Fernandez said the cause of the fire is still under investigation, adding "we cannot say whether it was arson."
The massive blaze that erupted on Saturday has killed 13 people and firefighters are still battling it, officials said Monday.