Indonesian police have arrested 72 people suspected of starting forest and peatland fires that have sent thick, unhealthy smoke across parts of the country and into neighboring Malaysia.
The fires have been fueled by a prolonged dry season and an intensifying El Nino weather pattern, according to Indonesia's National Disaster Management Agency, known as BNPB.
Forest fires are a recurring problem in Indonesia and have often caused tensions with neighboring countries.
Mohammad Irhamni, director of special crimes at the National Police's Criminal Investigation Agency, said plantation owners and traditional farmers often use fire to clear land for farming.
The suspects are being investigated in connection with 89 forest and land fire cases across five provinces on the island of Borneo and four provinces on Sumatra.
Most of those arrested are accused of deliberately burning land to prepare it for plantations at a lower cost. Investigators are also examining financial records to find out whether companies funded or ordered the burning.
"We will not stop at the perpetrators on the ground," Irhamni said at a news conference in Jakarta on Sunday. "We will determine who benefited from these activities and who was ultimately behind them."
The suspects could face heavy fines and prison sentences of up to 15 years, he said.
Thousands of hectares burned
Indonesian President Prabowo Subianto visited Central Kalimantan province on Saturday, where more than 7,600 hectares (18,700 acres) have been affected by fires.
Prabowo promised government support, including helicopters and water pumps, to contain the fires. He also vowed to take action against companies found responsible.
"I hope we can control this soon," Prabowo said, noting that Indonesia had faced even worse forest and land fires in the past.
Prasetyo Hadi, head of the State Secretariat, said at least four companies in West Kalimantan were being investigated for allegedly using fire to clear land. If they are found to have deliberately started the fires, they could face penalties including the loss of their operating permits.
Smoke covered parts of Borneo on Sunday. The island is shared by Indonesia, Malaysia and Brunei, with Indonesia controlling about three-quarters of its territory.
Indonesia's Environment Ministry recorded more than 3,700 hot spots across the island.
BNPB chief Suharyanto said fire activity had increased in West, Central and South Kalimantan. Fires in peatland areas are particularly difficult to control because flames can continue burning underground even after the surface appears clear.
He said heavy rainfall is needed to rewet the peat and completely put out the fires.
Haze reaches Malaysia
South Sumatra was also battling widespread fires, with more than 1,700 hot spots detected on Sunday. The resulting haze reduced visibility to just 10 meters (33 feet) in some areas.
Indonesia resumed cloud-seeding operations in Kalimantan, while 30 helicopters were deployed for water-bombing and aerial patrols. Another 22 helicopters were operating over Sumatra.
Malaysia has also been badly affected by the smoke, particularly the eastern state of Sarawak. One district recorded "very unhealthy" air quality on Friday, while eight others reported unhealthy levels.
The poor air quality led to the closure of nearly 600 schools and affected around 200,000 students, according to the Sarawak government.