Ngo Van Diem, 35, Nguyen Van Loi, 23, and Duong Van Hong, 53, poisoned a pine forest to clear land for cultivation, the police said.

They have also questioned several others and are expanding the investigation.

The forest used to supply wood for making paper and belonged to a company in the southern province of Dong Nai.

Earlier this month the pine trees suddenly began to wither for no apparent reason. It was then discovered they had been drilled and filled with poison.

The trees spread over 10 hectares (25 acres) in Lam Ha District, 73 kilometers (45 miles) from the resort town of Da Lat. The loss is estimated at VND800 million ($34,100).

The trees were almost 20 years old and had trunks 25-40 cm across.

The destroyed forest is surrounded by coffee and other farms.

Prime Minister Nguyen Xuan Phuc ordered the Ministry of Public Security to investigate the matter last month.

The Central Highlands lost nearly 358,800 hectares, or 14 percent of its forests, between 2008 and 2015, according to a report by the Central Highlands Steering Committee.

Most trees were chopped down by illegal loggers or cleared to make way for cash crops, roads or hydropower plants.

The country’s forests were disappearing so rapidly that Phuc ordered a total ban on clearing natural forests in 2016 to protect the remaining 2.25 million hectares.

In the three years since, while there has been a decline in deforestation, illegal logging and processing of forest products continue to occur in many places.