The financial watchdog will speed up its investigation into the alleged stock manipulation case amid investors' cry over unfair trading.

Shares in five companies -- Dongil Industries Co., Manho Rope & Wire Ltd., Pangrim Co., Taihan Textile Co. and Dongil Metal Co. -- plunged by a daily-permissible 30 percent Wednesday, prompting the country's bourse operator, the Korea Exchange, to suspend their trading starting Thursday.

The stock plunge came amid investors' woes over stock manipulation cases. For one, a slew of companies crashed for days earlier this year, causing massive losses to retail investors.

"We've handled the issue and the stocks in question since long ago, and we've also kept close eyes on unusual movements in stock prices, their causes and people concerned," Lee Bok-hyun, the head of the Financial Supervisory Service, said during a parliamentary session.

"The Financial Services Commission, the FSS, the prosecution and the KRX have joined hands to look into the case, and we will come up with results as soon as possible."

The authorities suspect the possibility of stock manipulation, as the items were all recommended on an online community suspected of a similar case before.

Police also raided the operator of the online community, and the prosecution imposed an overseas travel ban on him. (Yonhap)