A Swiss man who was detained in Frankfurt last week had allegedly spied on German financial fraud investigators in order to obtain information on how they catch tax cheats who hide their money abroad, according to media reports.

German prosecutors said the 54-year-old man, identified only as Daniel M. because of German privacy rules, was "suspected of working for the intelligence agency of a foreign power since the start of 2012," but declined to say which country was allegedly involved.

The Welt newspaper reported Sunday that man had been sent to Germany by Switzerland's NDB intelligence agency to identify German tax investigators involved in the purchase of confidential Swiss bank client data.

Germany has in the past bought the names of Swiss banking clients to determine if they were cheating on their taxes.

An NDB spokeswoman, Carolina Bohren, declined to comment, citing "the ongoing case."

The finance minister of the state of North Rhine-Westphalia, Norbert Walter-Borjans, said in a statement that Swiss spying against German investigators showed that Switzerland's financial industry had not yet changed its ways when it comes to protecting foreign tax evaders.

"The government of North Rhine-Westphalia and its finance minister will not be intimidated," he said.

The western German state has spent 19 million euros (20 million dollars) to buy several data troves on foreign bank accounts, which have resulted in belated tax payments and tax fines of 6.3 billion euros.