A probe into a missing loan of £10.25m to a football club is a “complex financial and corruption investigation,” police said.
Northampton Borough Council loaned the money to Northampton Town in 2013 and 2014 to redevelop the stadium, but the funds seemingly vanished.
Allegations of theft, fraud, money laundering, bribery and misconduct in public office are being investigated.
Northamptonshire Police said the public “may understandably find the length of the investigation frustrating”.
But they said the number of people being investigated was “higher than usual” and they were going through “vast amounts of data obtained from multiple sources”.
“It is necessary to build a robust case that results in both fair trials and safe convictions,” a police spokesperson added.
More than 550 witnesses and suspects have been interviewed in relation to the case.
The work on the stadium was uncompleted after contractors who were not paid pulled out.
The borough council has spent more than £1m on legal fees trying to claw back the missing loan, the BBC revealed last year.
The council apologised following a report by PricewaterhouseCoopers in 2016, which said the loan was rushed through without sufficient checks.
In January, Anthony Cardoza, and his son David, the former owners of the club were ordered to repay Northampton Borough Council £2.1m.
Anthony Cardoza was declared bankrupt in April after losing his appeal against repaying the money.
So far seven people have been arrested in connection with the police inquiry, including the former chairman David Cardoza, who is on bail.
A further 23 people have been interviewed under caution.