The Sun reports police taking part in an investigation into suspected fraud surrounding Mitrovic's move to Newcastle have made two arrests after a series of raids across Europe.
Authorities in Belgium have been conducting far reaching inquiries into suspected money laundering related to transfers in the country since earlier this year.
Mitrovic's £13million transfer from Anderlecht to Newcastle four years ago has formed part of their investigation, which saw the Belgian FA headquarters searched in April.
And over the past 24 hours, police and tax officials raided properties in London, Monaco and Belgium - with the Mitrovic deal said to be central to the development.
An agent was detained in Monaco, along with his assistant who was held in the Belgian city of Liege.
A statement from the Belgian prosecutor's office said: "The facts involve notably money-laundering operations and private corruption in the context of football player transfers."
Fulham striker Mitrovic, 24, is not thought to play any part in the investigation.