Some of these operators, a commonly used term for those manipulating stock market trades for personal gains, have been already barred by the regulator for their past misdeeds, but are suspected to be still operating through front entities.
Each of such operators tend to have their own 'portfolio' of select stocks and any movement in even of one of the constituents generally triggers a wide-based surge or fall in the entire portfolio, a senior official said.
The initial investigation into the last week's mid-cap carnage, when a number of stocks fell by 20-30 per cent and the loss was of more than 60 per cent within minutes in one case, has brought to Sebi's notice a synchronised pattern of manipulative activities and the regulator is further probing the matter to nail the culprits, he added.
There are suspicious that a rogue trader, who was barred from the capital markets last year after being found guilty of manipulating various stocks, could be behind the last week's crash in mid-cap stocks, although probe is yet to be completed, the official said.
Sebi is looking to strengthen its surveillance and inspection systems to guard against such operators and put their so-called portfolios under a greater scrutiny.
While there have been cases when such operators have been in league with the promoters or top executives of the companies whose shares are manipulated, but it has not been the case on all occasions.
The regulator suspects that these operators create panic by spreading negative rumours about the target stocks to hammer down the share prices, while there have been also instances of share prices being pushed up artificially through spread of unofficial positive speculations.
Sebi had initiated a probe on February 25 after many mid-cap and small-cap shares crashed within minutes without any obvious fundamental reasons.
It is suspected that most of the affected stocks are part of an 'operator portfolio' linked to a trader, who has been barred from the capital markets.
Initially, it was said that the stocks could have crashed because of large-scale sale of pledged shares in those companies, but most of them later claimed that none of the financiers had sold the shares encumbered with them. Some of the affected companies also later wrote to Sebi seeking a thorough regulatory probe in their share falls.
The regulators are also looking into the unusual trading volumes, besides highly volatile share price movements, of these stocks in the recent days.