The Serious Fraud Office has called for customers who bought tickets from Xclusive Tickets but never received their purchase to come forward.
The company went into liquidation in August after it failed to deliver nearly 18,000 tickets for a series of events, including the Reading and Leeds festivals.
Five people have been arrested and bailed in connection with the SFO's investigation into the company, which operated out of an address in Islington, North London.
During an appearance on BBC Radio 4's You and Yours programme, SFO director Richard Alderman said there was 4,000 people affected by the problem.
"We've put a simple form on our website which will enable people to put details of the tickets they wanted and the loss they suffered," he said.
Xclusive Tickets was one of spate of secondary market companies which failed to deliver tickets to fans over the summer.