Oct. 23--Two foreign brokers -- Credit Suisse and UBS -- and a prominent Thai investor have come under special scrutiny for large trading transactions made last week on the Stock Exchange of Thailand.

The SET index fell more than 7 percent over two days on investor concerns about the health of His Majesty the King, who has been in hospital since Sept 19 and is recovering from a lung infection. The index, which closed at 692.72 points on Oct 15, rebounded to reach 731.61 at the close last Monday after doctors issued public statements that His Majesty's health was stable.

The SET has since fallen over the past few days on concerns about the global economic recovery. The index closed yesterday at 708.76 points, down 7.59, in trade worth 19 billion baht.

Authorities have vowed to investigate whether there was any concerted effort to spread rumours for financial gain.

Thirachai Phuvanatnaranubala, the secretary-general of the Securities and Exchange Commission, said regulators were investigating unusual trades made by one wealthy Thai investor last week.

According to the SEC, short-selling volume totalled 461 million baht from Oct 7-13, of which one investor alone was responsible for 204 million. Short-selling is a trading technique used by investors in anticipation of a decline in share prices.

The unidentified investor also sold directly 28 million baht worth of stock, for total transactions of 232 million.

From Oct 14-16, the same investor purchased 52 million baht worth of stock to cover the short positions and purchased another 129 million directly.

Two foreign brokers, Credit Suisse Hong Kong and UBS Singapore, were also responsible last week for processing large trading orders that have attracted regulatory attention.

On Oct 14, Credit Suisse submitted sell orders for 3 billion baht worth of Thai stock on behalf of unknown clients while UBS Singapore sold 1.3 billion. On the next day, Credit Suisse submitted buy orders of 3 billion baht.

Sheel Kohli, a spokesman for Credit Suisse in Hong Kong, and Mark Pandit of UBS declined to comment.

The SEC also discovered that one Thai national residing overseas sold 219 million baht in shares on Oct 14, while on Oct 15, another Thai national purchased 145 million. Regulators said both transactions were not being treated as unusual. The SEC is seeking additional information from the SET and the Thailand Futures Exchange on trading transactions made during the period.