European and Portuguese police say they have dismantled a mainly Russian criminal gang laundering money through football in Portugal.
EU law enforcement agency Europol said the gang purchased football clubs and then used them as a front for an opaque network of holding companies.
It said four major football clubs were searched along with houses and offices, resulting in three "key" arrests.
Third division club Uniao de Leiria was among those raided.
Leiria's owner Alexander Tolstikov has been detained, along with two other club officials, Portuguese media report.
They are all due to appear in court on Thursday morning.
Uniao de Leiria was a top-flight club, at one point managed by Jose Mourinho, but it fell out of the Primera Liga in 2012 and dropped to the third division before going bankrupt and being bought by Mr Tolstikov in 2015.
It is currently fighting for promotion to the second division.
Europol said the searches and arrests were carried out on Wednesday as part of Operation Matrioskas (Russian dolls), which had been going on for more than a year.
Links were found with serious and organised crimes carried out in the UK, Germany, Moldova, Austria, Latvia and Estonia, it said.
Three other clubs were searched, top sides Sporting Lisbon, Sporting Braga and Benfica - but Portuguese media said they were not under suspicion and were only linked to the investigation because of football transfer negotiations conducted with Leiria.
Several Russian players have been on Uniao de Leiria's books in the past two years, but only one has been in the main squad, Reuters reports.
The gang behind the alleged club takeovers was "thought to be a cell of an important Russian mafia group", the agency added. According to Europol, the group adopted a particular method:
- Identifying EU football clubs in financial distress
- Infiltrating the clubs with benefactors who gained trust through short-term donations or investments
- Buying the clubs as a front for networks of opaque holding companies owned by offshore shell companies
- Laundering money through over-valuing or under-valuing players on the transfer market, buying TV rights and betting activities
Due to the use of front-men, the real owners who ultimately controlled the club were unknown, it said.
The investigation was triggered due to suspicions raised by "strong red flag indicators" - mainly the "high standards of living suspects enjoyed".