Police are to question the younger brother of politician Thanathorn Juangroongruangkit about his role in a case for which two people were jailed for taking bribes.
Sources said police investigators were studying a Criminal Court ruling from November 2019 in which Sakulthorn Juangroongruangkit was mentioned, although he was never charged with any offence.
Mr Thanathorn is a co-founder of the Progressive Movement.
The bribery case was sent to the Criminal Court for Corruption and Misconduct Cases, which in November 2019 convicted Prasit Aphaiphoncharn, a Crown Property Bureau official, and Surakit Tangwithoowanich, of taking bribes and forging documents while acting as land-lease deal middleman.
Both were found to have been in contact with Mr Sakulthorn, who is the chief executive of Real Asset Development.
The two men were said to have convinced Mr Sakulthorn they could help him bypass the Crown Property Bureau's land-leasing process in return for a payment of 20 million baht.
The court found them guilty of fabricating documents to try to trick Mr Sakulthorn into paying them money.
The court verdict dated Nov 27, 2019, read that Mr Sakulthorn had paid money three times to defendants. "The first payment is 5 million baht, and after receiving [forged] documents Mr Sakulthorn then paid 5 million baht and another 10 million baht to both defendants to process the land lease and pay the money to the deputy director of the Crown Property Bureau who is also state official, in a bid to influence this official to make a favourable decision in granting a land lease to the Real Asset Development company, without entering the official bidding process," according to the court.
The land at the centre of the deal is a prime 12-rai plot in the central Bangkok area of Chidlom, one of the capital's most expensive districts.
The two men were each jailed for three years without the possibility of parole, a sentence they did not appeal.
Sources said the original investigators decided to prosecute those two people first before going on to gather more evidence against other suspects, including Mr Thanathorn's brother.
The Central Investigation Bureau's legal department later ruled that the evidence collected so far could be used by police to take legal action against Mr Sakulthorn.
The Crime Suppression Division has now summoned Mr Sakulthorn for supporting the wrongdoing of a government official.
Democrat MP Watchara Petthong on Monday lodged a complaint with national police chief Pol Gen Suwat Jangyodsuk, asking why investigators had not already pressed charges against Mr Sakulthorn.
Nipit Intarasombat, deputy Democrat Party leader, asked on Facebook why Mr Sakulthorn had not been charged. "Mr Sakulthorn was mentioned in court as the person who provided the money. Is this going to be a repetition of the 'Boss' Red Bull heir case?" he said.