The Serious Fraud Office (SFO) has been investigating Hanover Finance for three months and says it has reasonable grounds to believe fraud may have been committed.
Hanover was owned by Mark Hotchin and Eric Watson and 16,000 investors had their investments frozen in July 2008.
In December last year, the company's assets and loanbooks were sold to Allied Farmers for $400 million.
SFO chief executive Adam Feeley says 30 people have now been called on to give interviews and produce documents.
Senior lawyers and forensic accountants are helping with the investigation and it will be a long and complex affair, he warns.
Mr Feeley told Checkpoint it will be a thorough inquiry which he believes will be in the interests of those under scrutiny as well as investors.
"Investors want something out of this - at the very least they want to know that there has been a thorough and exhaustive investigation. And if that takes six months or 12 months, then so be it."
Mr Feeley says it is not feasible for all aspects of the company's collapse to be investigated, so his office will focus on key areas such as the payment of dividends, transactions just before a moratorium proposal was put to investors and the eventual transfer of assets to Allied Farmers.
Allied Farmers on Monday announced it would pursue personal claims against former Hanover directors and executives, including breaches of duties.