Mr Nishi's dealings with the rogue trader have prompted interest and investigators are hoping to examine records held at the Tokyo offices of a former Japanese agency of UK-based Winchester Commodities, with whom Mr Nishi was involved. It is understood that Mr Nishi's home address is known to regulators, who will attempt to interview him in coming days.
The office in Tokyo trading as Winchester is based in the Minami Aoyama district, described as "fashionable and very expensive", home to embassies, nightclubs and boutiques.
Mr Nishi's name first surfaced in a fax sent by Yasuo Hamanaka to David Threlkeld, the metals trader who tried to blow the whistle on Sumitomo in 1991. Mr Threlkeld was asked to confirm non-existent trades with Sumitomo, backdated to the previous year.
The person to whom he was to send the fax was Mr Nishi, then said to be working at Winchester Tokyo. After Mr Threlkeld's business collapsed in the wake of his whistle-blowing activities, he sold his Tokyo operation to Mr Nishi for about $80,000 (pounds 50,000) in 1992.