EY and PwC are among the firms being investigated by the auditing watchdog over the London Capital & Finance (LCF) scandal in which pensioners lost millions of pounds.
The Financial Reporting Council (FRC) said it would question the two Big Four firms and Oliver Clive & Co about accounts they signed off.
EY and PwC each audited a set of annual results in 2016 and 2017, the FRC said. Oliver Clive audited the company for a month in April 2015.
Around £237 million of investors' cash was still inside the LCF scheme when it collapsed in January last year, leaving pension savers and many retirees out of pocket.
The unregulated company, which sold what the Financial Conduct Authority calls mini-bonds, is not covered by the Financial Services Compensation Scheme, which can pay out up to £85,000 to wronged investors.
However, the FSCS is currently investigating whether some investors may still be able to claim compensation because of the way the investments were marketed.
Around 11,000 savers who were desperate to get some return on their money, with interest rates at historic lows, pumped their cash into the LCF scheme.
Many have given up hope of ever seeing that money back, but they have showed great determination in holding people to account over the failure.
In January, hundreds of investors attended a meeting organised by Dame Elizabeth Gloster, a former Court of Appeal judge.
She is probing the actions of the Financial Conduct Authority over the scandal and is expected to report her findings later this year.
The news comes just a few months after Blackmore Bond, another mini-bond seller, collapsed with around £40 million trapped inside.
Many investors in LCF also put money into Blackmore.
