A statement from Northamptonshire Police said it was investigating "allegations of theft and fraud, bribery, misconduct in public office and electoral offences".
Northampton town received the loan from the local council in order to redevelop the Sixfields stadium.
Police say that they have interviewed a further nine suspects under caution.
Detectives would not comment on the "nature of the interviews or the identity of those interviewed", Thursday's statement said.
The issue first surfaced in the summer of 2013, when the loan was processed too quickly and with inadequate information and safeguards.