The owner of Rang Noi island, off Phuket's east coast not far from Maithon island, has seven days in which to produce a property ownership title and a legitimate explanation for the buldings.
About 60 officers from the Natural Resources and Environmental Crime Division in Bangkok landed on the island today with Forests Department officials.
Splitting into three teams, the officers set out to check structures on the island, see whether any encroachment had occurred on beaches and find any people on the island.
The division's Commander, General Narasak Hemnidhi, told accompanying media that officers suspected the motives of a American man who had paid 900,000 baht to lease the island for 30 years.
The man, named as John Kevin Baldwin, had connections to a casino in Laos, the general said. He was allegedly an owner of the Savan Vegas Casino in Savannakhet province.
The island's largest building of two storeys was about 20 percent complete and had an upper floor with views out over the surrounding seas.
The large space below was enclosed and officers found a walk-in safe there.
In another building, investigators found two speedboats and three jet-skis.
Complaints from residents who lived on nearby islands had led to the investigation, the general said.
The general said a sor kor one title had been issued for the island's 17 rai back in 1954 but in 1969 nobody was living on the island so the sor kor one was suspended.
In 1975, two people found living on the island claimed ownership of 14 rai.
In 2007, a chanote title for 15 rai was issued to Bridge Rang Noi company.
In March 2012, Kan Krao company bought the land. The company was represented by a 32-year-old Thai man from Korat.
Mr Baldwin, a shareholder of Bridge Rang Noi, subsequently leased the island for 30 years.
The general said he believed the Kan Krao company may have been a nominee for Bridge Rang Noi.
Anyone with claims to ownership of the island has seven days to present themselves, after which the matter wil be handed over to police.
A lawyer for Kan Krao who went with the group said the company could respond to prove everything it had done was legal. The buildings were not for a casino, he said.