Financial authorities have decided to suspend trading for shares of five companies suspected of stock manipulation.

According to the Korea Exchange(KRX) on Wednesday, shares of four companies listed on the primary KOSPI market and one on the tech-heavy KOSDAQ fell sharply around the same time, beginning at 11:46 a.m., causing losses of some 500 billion won, or nearly 400 million U.S. dollars, in 30 minutes.

After the losses, the Financial Services Commission, the Financial Supervisory Service and the KRX said that the trading of shares will be suspended from Thursday for Dongil Industries, Manho Rope & Wire, Pangrim, Taihan Textile, and Dongil Metal.

Financial authorities said they are looking into the possibility of unfair trading practices related to the listings.

The incident has sparked suspicions of such activity in light of a similar large-scale stock manipulation case in April that triggered huge losses and rocked the local stock market.