The investigation follows a complaint lodged yesterday with the Natural Resources and Environmental Crime Suppression Division by Atchariya Ruangrattanaphong, chairman of the Help Crime Victims Club.
"These politicians and senior authorities may have illegally acquired the land in the same way Palang Pracharath MP Pareena Kraikupt did. The MPs are members of both government and opposition parties," Mr Atchariya told the media yesterday (Dec 9).
They include MPs attached to the Palang Pracharath, Democrat, Bhumjaithai, Pheu Thai and Future Forward parties.
Among the five senators is a former member of the now-dissolved National Reform Council (NRC), while another four are high-ranking officials.
One MP, a political party heavyweight in the government camp, has 120 rai of so-called Por Bor Tor 5 land in Prachuap Khiri Khan, but it is not clear whether he acquired the plots legally, according to Mr Atchariya.
Ms Pareena earlier officially declared assets with the National Anti-Corruption Commission (NACC) which included 1,700 rai of land in Ratchaburi province.
The plots, according to Ms Pareena, have Por Bor Tor 5 state land status and she paid the appropriate taxes in exchange for the right to use them.
However, the MP has been accused of illegally acquiring 682 rai of Sor Por Kor land and encroaching on 46 rai of state forest in Ratchaburi.
The Agricultural Land Reform Office (Alro) yesterday rejected Ms Pareena Kraikupt's letter of intent to return the Sor Por Kor land.
Ms Pareena had submitted the letter to deputy agricultural minister Capt Thamanat Prompow.
Capt Thamanat told media that her letter is not acceptable.
"Her letter of intent contains a clause that might lead to legal conflict in the future. We have asked her to submit a new letter within seven days. We don't want any further legal problems," Capt Thamanat said during a visit to Ms Pareena's chicken farm in Ratchaburi's Chom Bueng district.
In the letter, Ms Pareena stated that she would like to preserve her right to continue her chicken farm business on the office's land when Alro redistributes the land back to farmers, according to Capt Thamanat.
"Her request is against the office's land allocation regulations as she is not a poor farmer," he said.
According to her asset declaration, Ms Pareena's net worth is 163 million baht.
Regarding the legal action against her, he admitted there is an "overlap" issue between the Royal Forest Department under the Ministry of Natural Resources and Environment and Alro, under the Ministry of Agriculture and Cooperatives.
The public forest land which Ms Pareena has occupied was once under the jurisdiction of the RFD before later being transferred to Alro in 2014.
Capt Thamanat said he would ask both offices to form a joint committee to come up with a clear policy over which has the authority to take legal action that can be applied to similar cases that may arise countrywide.
The need arises as there has been no clear precedent established, he said.
In some previous cases, the Supreme Court had handed down verdicts that the land is still under forest law, said Capt Thamanat.
However, the Office of the Council of State has come to a different conclusion in other cases and found that the land is under Alro because that is the department the transfers were handled by.
The Department of Royal Forest's legal team yesterday had a meeting to discuss whether to file further legal action against Ms Pareena over the 682-rai plot which later had been transferred to Alro during the period her farms were operating on the plot.
The RFD has already charged Ms Pareena with encroaching on 46 rai of public forest. If found guilty, she faces a jail term of up to 20 years.
Nantaya Boonyanan, the department's spokeswoman, said that the meeting agreed to ask the Office of the Council of State to verify whether the department was authorised to take legal action against Ms Pareena over the plot.
"We want to make sure that we have the authority to do so. If not, we might be held accountable for overstepping our authority," she said.