"Aerial pictures suggest that these properties are located inside the Khao Phu Luang forest reserve," the office's director Suthep Pawareswittayararn said yesterday.
However, he said his office would investigate the case further before deciding on whom it should lodge a complaint against.
Relevant authorities have now been seriously surveying the Khao Phu Luang forest reserve in a bid to crack down on encroachment.
Police investigators have already summoned the owners of 22 other properties located in the forest reserve to acknowledge charges against them.
Of these 22, six have come forward but denied the charges against them. They have produced evidence of localmaintenance tax payment, and photos showing that the plots they were occupying were degraded forestland.
Pol Colonel Wachirawat Kritrittisak, an investigator for these encroachment cases, said a second summons would be issued for the owners of the 16 other encroaching properties.
"If they still don't report to us, they will face arrest warrants then," he said.
On the encroachment on the Tab Lan National Park, Suthep said that the chief of the National Parks, Wildlife and Plant Conservation Department would lead a team in demolishing three encroaching properties on September 21.
One is located in Wang Nam Kheow, while two others are located in Prachin Buri's Na Di district.
"The demolition is to be carried out under court orders," he said.