Pisit Leelavachiraphat, director of the OAG's legal division, said the office would try to finish the inquiry next month to give society what it wants to know - whether deals by Panthongtae and Pinthongta Shinawatra, Ample Rich Investments and Temasek Holdings are liable for tax.

"So far, we are satisfied with the Revenue Department's cooperation. Two officials are to due to meet us next week," he said.

Revenue Department director-general Sirote Swasdipanich went to the OAG yesterday to explain why the department did not tax Panthongtae and Pinthongta for the purchase of Shin Corp Plc shares from Ample Rich - before selling the shares to Temasek days later.

Sirote said after the three-hour session that the OAG had questioned the Revenue Department's operations as a whole, not only on the transactions. "I insist the Revenue Department has done its job within the legal context," he said.

Panthongtae and Pinthongta bought 329.2 million Shin Corp shares at Bt1 apiece and sold the shares at Bt49.25, netting a total of Bt15.88 billion. Had they been taxed, they would have been subjected to Bt5 billion in tax payments - based on the maximum 37 per cent personal income tax rate.

Auditor General Jaruvan Maintaka said yesterday the OAG may contact the Securities and Exchange Commission for more information.

"We need to get all the information, as this involves the Revenue Code. We can't proceed with recklessness," she said.

Pisit said the office should also finish the investigation into the bomb scanner procurement for Suvarnabhumi Airport next month.