The authorities are scrambling to respond to what could be one of the largest stock price manipulation cases in recent years.

The authorities, including the Financial Supervisory Services, are investigating deals involving the stocks of eight companies, the combined market capitalization of which fell around 8 trillion won this week.

The shares of the companies -- Samchully, Daesung Holdings, Seoul Gas, Sebang, Sun Kwang and Daou Data – began dropping abruptly earlier this week without any apparent financial cause. The shares dropped by 30 percent – the maximum daily change allowed in the Korean market – for up to four consecutive trading days, fanning suspicions of stock price manipulation.

The Financial Supervisory Service has launched an investigation into the case, raiding the office and related residences of a company that is suspected to be involved in the crisis on Thursday.

“A strict response to unfair trade is a basic principle in securing market credibility,” Lee Bok-hyun, head of the Financial Supervisory Service, told the press on Friday.

“We will investigate the case immediately and strictly, regardless of one’s status, assets or social position,” Lee said, referring to how Kim Ik-rae, executive chairman of DaouKiwoom Group, is suspected of being involved in the stock manipulation as he sold off his shares in Daou Data shortly before the stock prices plummeted.

DaouKiwoom Group denied the suspicion. Hwang Hyun-soon, CEO of Kiwoom Securities, said he will stake his post to guarantee that Kim is not related to the alleged stock manipulation charge.

The drop in the prices of the eight different stocks resulted from a mass selling spree of contract for differences arrangements through France-based finance firm Societe Generale. A CFD is a financial derivative arrangement that pays the difference in settlement price between the opening and closing of a trade, without actually owning the stocks.

In response to the crisis, securities firms, including Samsung Securities and Korea Investment & Securities, have closed new deals on CFD trading. Both firms only allow customers to liquidate their existing CFD accounts. Mirae Asset Securities and other firms have raised the bar for good faith deposits to 100 percent.

The drop in the shares of the eight companies has resulted in losses of market cap worth nearly 8.2 trillion won ($6.12 billion).

It was the first time listed shares have hit the maximum daily drop of 30 percent for four consecutive days since the daily price change limit was set at plus or minus 30 percent in June 2015. The share prices of the eight firms have started to recover from Friday.