By Tim Mansel
BBC World Service
When German police announced last November that they'd arrested 15 people whom they suspected of having fixed more than 200 football matches in Europe over the previous few months, many commentators muttered darkly about this being the tip of the iceberg.
Arif Erdem, pictured during his playing days, denies the match-fixing claims
That iceberg has now become slightly more visible after the police in Turkey this week arrested more than 40 people, also in connection with match-fixing.
They included several big names in Turkish football, including the former international Arif Erdem.
Erdem, who now coaches at the Turkish premier league club Istanbul Buyuksehir Belediyespor, has since been released and denies the allegations. Among those now charged, according to Turkish websites, is Recep Ozturk, a goalkeeper who plays for Konyaspor.
The BBC has seen German police documents that indicate a very clear link between the German and Turkish investigations.
From what we know about the German police investigation, it relies primarily on information gathered by listening in on telephone conversations between members of what the police say is a match-fixing ring based in Germany.
Ante Sapina: Accused of involvement in betting on fixed Turkish matches
One of the ringleaders is believed to be Ante Sapina, who first came to police and public attention in 2005, when he was jailed by a court in Berlin for bribing a referee, Robert Hoyzer, to fix games. Ante Sapina was one of those arrested in November.
One of the 200 games that came to the attention of the German police was played between Istanbul Buyuksehir Belediyespor and Genclerbirligi Ankara in Turkey's top division in May last year.
The BBC has seen transcripts of telephone conversations between members of the ring as they discuss their betting strategy in the hours leading up to the game and they make for fascinating reading.
According to these documents, the gang members are tipped off early in the week of the game that the result of the match has already been agreed.
This and another game in the same league are "200% certain" they are told. They are initially unsure of the quality of the information, and one of them remembers losing a lot of money betting on Turkish games in the previous year.
However, they are finally convinced to act. According to the police, one of those who makes money from this game is a Recep Ozturk, who was at the time with Genclerbirligi. He places money on a bet that his own team will lose.
Jacket full of cash
Some of those arrested in Germany last year are of Turkish origin.
Shortly after those arrests last November the lawyer of one of them gave details to the German press of the allegations against his client.
One was that he had bet on a game in the Turkish first division in April 2009, knowing that it had been fixed in advance, and that he had won 18,500 Euros as a result.
Another of those arrested, also of Turkish origin, made a statement to the German police last December, a copy of which the BBC has seen.
He said he was hoping to be granted bail because his girlfriend was expecting a child. In his statement, he alleges among other things that he was sent to Turkey by Ante Sapina with a jacket containing 100,000 euros in cash to give to contacts there; he also admits to allowing Ante Sapina, after his release from prison, to open a betting account in his name.
Charges have yet to be brought against any of those arrested last November. The prosecutors in Bochum, Germany, are giving little away, but say that some of the 15 have now been released on bail.
When we asked when charges were expected, we were told to ring back in two months. But with this investigation involving at least nine countries, even with this week's events in Turkey, it seems likely that most of the iceberg is still concealed in deep water.
You can hear a full account of the investigation by Tim Mansel and David Goldblatt on next week's edition of Assignment on the BBC World Service.