The Securities and Exchange Board of India has started an investigation into the dramatic 213 point fall in the Bombay Stock Exchange Sensex on Tuesday.

Sebi officials said the market regulator was trying to ascertain the reasons for the fall, since this was the largest after the market went down by around 270 points in March 2001, a day after the Union Budget was announced.

"We want to know whether there was any concerted attempt at bringing down the market or whether it was just panic selling. A 200 point movement definitely calls for a more closer scrutiny," they said.

Sebi has already asked the stock exchanges to furnish stock data, especially in the index stocks. Sebi has also asked for data on derivatives positions since there was large-scale unwinding in this segment. "Some unwinding is to be expected ahead of the expiry of the series, but we have to ascertain that it was not unusual," Sebi sources pointed out.

Some of the high volume counters such as Gail and ICICI Bank, where the open interest positions fell drastically on Tuesday, are being investigated by the Sebi surveillance team. However, there was also an increase in the open position on the ONGC counter.

Sebi is looking at the huge overbought positions in individual stock futures. Incidentally Tuesday's sell-off was also due to some five or six large hedge funds, which are investing in Indian equities through participatory notes, offloading their positions.

Sebi had also sought details of the transactions of these hedge funds, sources said.