Capital World Markets (CWM) is also being investigated by City of London Police for money laundering and false representation, the force said.
The firm, based in the City's Heron Tower, was raided on 3 March and 10 men and three women were arrested.
CWM's website has been removed and it could not be contacted for comment.
Computers and documents were seized during the raid, said the City of London Police, and the 13 suspects were questioned and released on bail until September.
Det Supt Maria Woodall said: "The primary objective of the arrest phase of this investigation was to stop what we believe was ongoing criminality and prevent people putting their money into CWM's managed funds offering 5% interest per month."
She said she needed people to come forward to see how the firm used the funds.
A trading company partnered with Chelsea Football Club is under a police fraud investigation, which has so far involved 13 arrests.