Prosecutors are preparing to question senior executives at scandal-hit Japanese internet firm Livedoor.
Maverick Livedoor boss Takafumi Horie is under investigation
Outspoken boss Takafumi Horie faces allegations that he was involved in breaches of securities laws, Tokyo's Nihon Keizai business daily reported.
Prosecutors also plan to quiz Livedoor executives Ryoji Miyauchi and Fumito Kumagai, the Kyodo news agency said.
Reports of the fraud investigation at Livedoor brought chaos to Japan's stock market earlier in the week.
Although the Tokyo Stock Exchange has recovered much of its losses, the market was forced to close 20 minutes early for the first time in its history on Wednesday as news of Livedoor's troubles sparked heavy selling in shares.
The benchmark Nikkei index closed up just 0.41 of a point at 15,696.69 on Friday.
Separately, the Tokyo Stock Exchange announced that from next week it would increase the capacity of its computerized trading system to five million transactions, up from the current limit of four and a half million.
Wednesday's unprecedented early halt to trading was triggered after the number of transactions traded on the day passed four million, taking the system close to its limit.
Shares in Livedoor, which have been hit by a glut of sell orders, have lost more than 50% of their market value since prosecutors raided the firm's Tokyo offices on Monday.
The company, which has denied it broke market rules by giving misleading information to shareholders, is due to hand over an internal investigation to the Tokyo Stock Exchange later on Friday.
Livedoor could be delisted from the exchange if it is found to have been in violation of Japanese securities laws.
On Thursday, the head of an investment firm involved in Livedoor's suspect business deals was found dead on the island of Okinawa in a suspected suicide.
The offices of HS Securities were searched on Monday as part of the probe into Livedoor, but the company has insisted it has done nothing wrong.
The developing scandal at Livedoor has raised concerns about the strength of Japan's new tech boom.
Critics worry that the steady sales-focused foundations upon which many of Japan's companies are traditionally built are being overlooked in a new world of rapid expansion fuelled largely by takeovers.
One of Japan's best known internet companies, Livedoor had grown rapidly through a series of acquisitions and stock splits into a group with a value of about 730bn yen ($6.3bn; £3.6bn) before the scandal erupted.
Mr Horie is a celebrated personality in Japan, where he is known for his critical views of the country's business establishment.
The 33-year-old shot to fame following separate failed attempts to buy a TV company and a baseball team.