Prosecutors have slapped a travel ban on an operator of an internet café on stocks on suspicion of involvement in the simultaneous fall of shares of five listed companies on Wednesday.

The Seoul Southern District Prosecutors’ Office said on Thursday that they placed the ban on the man in his 50s, surnamed Kang, whose online café has some six-thousand members.

According to prosecutors, Kang is a minority shareholder of some of the five firms that showed questionably concurrent drops in share prices that triggered losses of some 500 billion won, or nearly 400 million U.S. dollars, in 30 minutes.

The timing led prosecutors and financial authorities to suspect irregular trading practices affected the stocks of Dongil Industries, Manho Rope & Wire, Pangrim, Taihan Textile and Dongil Metal, four of which are listed on the primary KOSPI market while one is on the tech-heavy KOSDAQ.

Financial authorities launched an emergency inspection Wednesday as the incident shared similarities with a large-scale stock manipulation in April that triggered huge losses and rocked the local stock market.