The Royal Forest Department is investigating which state agency encroached on land in Suthep-Pui National Park, Chang Mai, by building a road to the so-called "Hi-So Hill", a sought-after area among wealthy people and property developers.
The scenic hill, located along the Sa Meng-Chiang Mai road in Hang Dong district, is under the spotlight since department chief Chonlatid Suraswadi on Sunday made an aerial inspection of parts of the forest area on the hill, some of which are believed to be occupied by wealthy businessmen.
Hi-So Hill is currently under close watch by forestry officials, police, soldiers and authorities from the Department of Special Investigation.
Five of the 30 resorts operating there are thought to have acquired their land title deeds illegally.
His initial check found at least five plots of land with properties had no Sor Khor 1 documents, which are used to apply for land title deeds.
"Some plots are simply held without any land documents," Mr Chonlatid said.
"Some plots have land title deeds, but the documents don't cover the whole area they claim rights to."
As for the road, Mr Chonlatid said "we still don't know who built it," adding that officials are checking whether it was built by a state or private agency and whether it is linked to the 30 resorts and condominiums.
He has ordered forestry officials to carry out more probes and continue the inspection, especially in the Mae Tha Chang Forest Reserve and Mae Khanin in Hang Dong district.
Mr Chonlatid's team believes there must be more than 10 plots of land allegedly acquired illegally in the forest reserve.
"I will file a legal action against them within 90 days if the land owners cannot present their land ownership documents.
"We might ask the Department of Land to verify whether there is a way of obtaining the land documents legally," he said.
The alleged irregularities there are among 80 suspected cases of forest encroachment in the northern province.
Nationally, there are up to 930 similar cases, which are mostly in and around the Central Plains, Mr Chonlatid said.
Examples are Kanchanaburi, Phetchaburi, Nakhon Nayok and Phetchabun, which are being looked into by the Phayak Phrai, a forest guard unit under the department.
The Phayak Phrai on Monday found additional alleged irregularities involving the five troubled plots of land.
Their land title deeds were issued between 2006 and 2009 and they claim ownership of parts of an area declared as a forest reserve in 1967, Phayak Phrai chief Cheevapab Chivatham said Monday.
The owners also do not have Sor Khor 1 documents with which they could ask officials to grant them title deeds, so "we believe it is an illegal issuance", he said.
Sor Khor 1 proves that a person has already informed officials they have occupied a certain plot of land.
Officials do not want to jump to the conclusion that all of the cases involve illegal land acquisition. They are checking land documents and asking land owners to clarify how they acquired those plots of land, said Mr Chonlatid.
If they are eventually found encroaching on forest areas, authorities will consider a lenient approach against only landless villagers, provided that they only farm the areas, Mr Chonlatid said.
"But we'll never grant this special condition to people who run other businesses," he said.