The Forestry Department has lodged a complaint with police against an elder sister of ex-monk Sompong Nakhonthaisong for encroaching on state land in Chaiyaphum.
Surachai Achalaboon, director-general of the Forestry Department, said he conducted initial inspections of some of the plots of land that may be illegally owned by the ex-monk.
Mr Sompong allegedly spent 10 million baht to purchase 300 rai of Sor Por Kor (agricultural land reform) plots in violation of the law.
The action follows activist Srisuwan Janya, secretary-general of the Association for the Protection of the Thai Constitution, filing a petition to the Royal Forest Department (RFD).
The petition called on the RFD to look into claims that 300 rai of Sor Por Kor land purchased by Mr Sompong when he was still a monk was in violation of the law.
The ex-monk said he was 10 million baht in debt due to purchasing the land so that his parents could plant rubber trees there.
A team of rangers, police, Natural Resources and Environmental Crime Division officers, the village head of Thung Na Lao and the deputy district chief of Khon San visited the site.
They found the first lot, or about 95 rai of land, was not owned by Mr Sompong but by his elder sister Kittiya Namwan.
They found that Ms Kittiya had built a house on a 26-rai site she purchased in plots from six owners.
In addition, she also built two barns and established rubber plantations, bamboo as well as other trees. She also fenced the land with barbed wire.
The authorities have seized the land and its assets.
Ms Kittiya will face charges of encroaching on state land and violating forestry regulations.
Mr Surachai said as Ms Kittiya owned more than 25 rai of land illegally, she is considered an investor, not a farmer.