Speculation over the deal, which saw Prime Minister Thaksin Shinawatra's family and relatives sell their 49.6 percent stake to a group led by Singapore's Temasek Holdings, had pushed Shin shares up 25 percent since December.

The Stock Exchange of Thailand launched its probe into the dealings after the deal was announced Monday.

"At the moment, we have not found any signs of insider trade in Shin Corp shares," SET president Kittiratt Na Ranong told reporters. "This deal is big. The exchange normally checks out whether people involved in such a deal took advantage of the information they had," he said.

ADVERTISEMENT

He did not say when the investigation would finish.

Shin shares ended unchanged at 48.25 baht (HK$9.58) Wednesday after hitting a 12-year high of 48.75 baht Tuesday. The overall stock market was up 2.25 percent.

The deal was done through the exchange in a series of transactions by SCB Securities, UBS, KGI Securities, Trinity Securities, Phatra Securities and Thanachart Securities.

The bourse has asked the brokers for information about all trades involving Shin shares.

The trading firms have not commented on the probe. Getting a conviction for stock manipulation or insider trading can take years.

Two cases of stock manipulation were prosecuted in Thai courts in 2005, with executives at two small companies receiving both fines and prison sentences.

But the convictions came five to six years after the investigations began.

Analysts said the complexity of the Shin deal would make it hard to prove that any insider trading had occurred.

"It's difficult to accuse someone of insider trading in Shin shares. Many brokers helped to perform the deal, so the orders came from different places," said one analyst.

Shin Corp said in the weeks leading up to Monday's announcement that it knew nothing about a deal, which had been the subject of rumors.

Critics said the exchange should have pushed the firm harder for information, but Kittiratt said the same standard was applied to Shin Corp as to all listed companies.

"There is no double standard. We asked Shin five times for information regarding the rumors and then informed investors accordingly," Kittiratt said. REUTERS