Prosecutors in Germany's most populous state are investigating 57 foreign banks on charges of abetting tax evasion, according to the "Süddeutsche Zeitung." The banks, from Switzerland, Liechtenstein, Austria and Luxembourg, have all been under suspicion over the past two years, the report said on Tuesday.

The banks were brought to the investigators' attention thanks to a wave of German citizens offering voluntary declarations of their foreign bank accounts over the past six years - concentrating in particular on banks mentioned repeatedly.

The German authorities have already negotiated fines with banks in around 10 cases, according to the paper, amounting to a total of around 120 million euros ($135 million). The biggest single fine came from the Basler Kantonalbank, Switzerland, which handed over 37.1 million euros.

The Luxembourg bank BCEE has paid 14 million euros, while the Deutsche Bank in Switzerland has paid some 10 million euros. On top of that, in one particularly egregious case, a bank is believed to be negotiating a fine of around 60 million euros.

The Austria-based Akte Walser Privatbank confirmed that it had paid its fine of 5.4 million euros, and that its policies had become tighter since the case. "Our bank changed the way it deals with foreign customers radically in 2009," a spokesman told the paper. "The requirements for the identification of customers and the transparency about the origin of the means are very strict."

BCEE did not respond to a request for comment, while a spokesman for Deutsche Bank in Switzerland told DW that it never comments on such cases.

New methods

Paying closer attention to voluntary declarations has become German prosecutors' new favorite method of tracking down tax evaders.

Instead of buying up CDs of customer data from disloyal bank employees (or other illicit sources) - a move that divided Germany's political parties when authorities spent several million euros to do so in the past - the authorities now increasingly investigate banks that repeatedly appear on voluntary declarations.

The former tax evaders are then questioned for more information about the banks. In an email to DW, however, a spokesman for the Cologne state prosecutors said they could "neither comment on or confirm" the "Süddeutsche Zeitung" report and that they did not record statistics about voluntary declarations.

Tenacious taxmen

North Rhine-Westphalia has become Germany's most vociferous tax crime prosecutor in recent years.

The state has acquired 11 data storage devices since 2010, which according to a statement from early August, created a wave of voluntary declarations and fines that has brought some 6 billion euros to Germany's public coffers.

The most recent example was in August, when someone anonymously (and free of charge) sent the NRW Finance Ministry an external hard drive containing the details of some 160,000 bank accounts - 54,000 of which were German, around 50,000 Belgian, and around 42,000 French. State Finance Minister Norbert Walter-Borjans has pledged to pass on all the data the state has to the relevant foreign authorities across Europe.

The latest revelations show that German prosecutors are now paying particular attention to banks in Austria, where banking secrecy was once protected by the constitution. In 2014, the European Court of Justice ruled that this was unlawful, triggering a sharp rise in voluntary tax declarations, particularly in Bavaria - while many Austrian banks let employees go.

Last week, various German media outlets reported that the NRW tax authorities were also pursuing major multinational financial institutions, including J.P. Morgan, Barclays and HSBC, on suspicion of "devious stock trading," allowing them to avoid some 10 billion euros of tax over several years.

"Our tax investigations go after every suspicion - without regard for either individuals or credit institutions," Walter-Borjans said. He added that a number of banks had already entered into negotiations and were cooperating with authorities, though he would not go into details. He went on to urge other banks to follow suit.