Security Minister Luis Cordero said the suspect used a liquid accelerant to start fires in a wheat field, with authorities seizing five liters (more than a gallon) of fuel from him.
He was arrested at dawn in the town of Perquenco in Araucania region, south of Biobio.
The fires began simultaneously on Saturday in various parts of Biobio and Nuble regions, about 500 kilometers (310 miles) south of the capital Santiago.
Fanned by strong winds and high temperatures, the flames quickly ripped through the coastal towns of Penco, Lirquen and Punta de Parra, leaving a blackened landscape of smoldering ruins.
Around 1,000 homes went up in smoke, affecting around 7,500 people.
Other fires were later reported further inland, in the Biobio town of Florida, about 40 kilometers east of the city of Concepcion and in Araucania.
Cordero said substances used to start fires, including plastic containers containing accelerant, were found in Concepcion.
Firefighters were still battling 35 blazes Wednesday -- 22 in Biobio, five in Nuble and eight in Araucania, according to national forestry officials.
A drop in temperature in recent days has helped ease the situation.
"We managed to reduce the intensity of the fire," Carlos Zulieta, a firefighter in Florida told AFP, adding that it was now advancing "more slowly."
The government said it would pay compensation of $700 to $1,500 to victims.
Aid began trickling into affected areas on Wednesday.
Municipal workers and private companies were delivering portable toilets and generators to Lirquen, where some families are camped out in the ruins of their homes.
In February 2024, wildfires broke out around the coastal resort of Vina del Mar, 110 kilometers from Santiago, leaving 138 dead.
Investigations revealed that firefighters and forestry brigade members started the fire, which spread rapidly due in part to high temperatures during the southern hemisphere's summer.
Chile blaze victims plead for help from razed neighborhoods
Lirquen, Chile (AFP) Jan 21, 2026 -
Residents of southern Chile pleaded for help Tuesday from communities ravaged by four days of deadly wildfires, as smoke lingered and firefighters braced for the return of hot weather.
The blazes have killed 20 people and razed entire neighborhoods in Chile's Biobio and Nuble regions, consuming an area the size of the US city of Detroit.
Officials believe some of the fires were intentionally set and said one person had been arrested on suspicion of attempted arson in the coastal town of Penco.
More than 7,200 people have been affected, authorities said, and despite official promises of aid, residents told AFP they were getting help mainly from local Samaritans.
"I tip my hat especially to people of modest means, because they're the only ones who came all the way out here. I have yet to see the state step in to help," said Manuel Hormazabal from the town of Punta de Parra, about 30 kilometers (19 miles) from Concepcion, the regional capital of Biobio.
"We're lacking the essentials here, which is electricity and portable toilets," said the 64-year-old gravedigger, who now sleeps in a tent with his children, grandchildren and two dogs.
All that remains of his home is a bathroom that cannot be used.
Against the backdrop of charred cars and the remnants of houses, others shared his frustration.
"It's only the people helping the people. There's no one else. It's only now that some officials are showing up, but the aid is minimal," said Maria Jose Pino, a 23-year-old veterinarian from Lirquen, one of Biobio's hardest-hit towns.
Deputy Interior Minister Victor Ramos told reporters Tuesday that "all processes are being accelerated so that aid reaches residents as quickly as possible."
Measures include financial assistance ranging from $350 to $1,500 per family.
- 4,000 firefighters -
Cooler weather offered a respite after three scorching days, and nearly 4,000 firefighters remained mobilized Tuesday to battle 21 fires that were still active in the regions of Nuble, Biobio, and neighboring Araucania.
"The fires are advancing more slowly," firefighter coordinator Juan Quevedo told AFP, attributing the trend to lower temperatures, higher humidity and a shift in wind direction.
But he warned intense heat is forecast to return within days.
Hundreds of homes were destroyed or damaged as the fires ripped through hilltop districts in Penco and the adjacent port of Lirquen, leaving a blackened landscape of smoldering ruins.
The fires tore tin roofs off homes, blew out windows, and charred cars.
Nataly Gonzalez, a 41-year-old living in the center of Penco, said her home escaped damage but relatives living in Lirquen, who "lost absolutely everything," had yet to receive any state help.
She said her relatives were trying to clear rubble and ash from their homes "without helmets and with burnt tools."
Identification of people who died in the blazes has been slow, and just five victims had been named by Tuesday.
Many bodies were unrecognizable.
Wildfires have severely impacted south-central Chile in recent years, especially in its warmest and driest summer months of January and February.
This year's blazes are the most severe since February 2024, when several fires broke out near the city of Vina del Mar, northwest of Santiago, resulting in 138 deaths.
A 2024 study led by researchers at the Santiago-based Center for Climate and Resilience Research found climate change had "conditioned the occurrence of extreme fire seasons in south-central Chile" by contributing to a long-term drying and warming trend.
Argentine Patagonia was also struck last week by violent forest fires that ravaged more than 15,000 hectares, according to local authorities.
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