The head of scandal-hit Japanese internet firm Livedoor has been arrested amid allegations that he broke stock market rules.
Prosecutors have turned up the heat under Livedoor's Mr Horie
Three other executives were arrested alongside Livedoor boss Takafumi Horie, 33, who has denied the allegations.
Livedoor's problems have shaken Japan's business world and stock market amid fears that more problems may emerge.
The allegations were central to last week's share sell-off that forced the Tokyo stock market to close early.
The company is accused of giving misleading information to shareholders.
If found guilty, Livedoor could be delisted from the stock market.
Already, the firm's shares have tumbled by nearly 65%, knocking close to $4bn (£2.2bn) off its market value.
"Talk in the market about Livedoor going bankrupt or being broken up and sold is strong," the Daiwa Institute of Research said in a report.
One of Japan's best known internet companies, Livedoor had grown through a series of acquisitions and stock splits.
Mr Horie is a celebrated personality in Japan, and is known for his critical views of the country's business establishment.
He has repeatedly, and vehemently, denied any wrongdoing.
"I have no recollection of doing anything we are under suspicion for," Mr Horie wrote recently on his internet site.
On Monday, Livedoor's shares dropped by 80 yen to 256 yen, the largest share move allowed in one day.