The chief executive of Deutsche Post, the firm that owns global delivery firm DHL, has been quizzed by police over suspected tax evasion.
Mr Zumwinkel (left) has had his home searched
Klaus Zumwinkel had his home and office raided by investigators and was questioned before being bailed.
Prosecutors said he was suspected of not paying 1m euros (£750,000; $1.4m) in taxes using banks in Lichtenstein.
Mr Zumwinkel is one of Germany's most influential business figures, having headed Deutsche Post for 18 years.
A company spokeswoman confirmed that an investigation into its chief executive had been launched but did not specify what it was about.
Mr Zumwinkel has led Deutsche Post since 1990 and oversaw its flotation in 2000.
As Mr Zumwinkel is approaching retirement age, many analysts do not expect him to have his contract renewed when it expires later this year.
And a spokesman for the Social Democrats, who form part of Germany's governing coalition, said that if Mr Zumwinkel was found to guilty, he should "not hold his job for one minute longer".
"It is beyond comprehension that a multimillionaire like Zumwinkel found it necessary to save a couple of million euros in this way," Rainer Wend told Der Spiegel magazine.
The Deutsche Post boss has been praised for helping transform the firm into a logistics giant, as well as developing its retail banking arm, Postbank.
However, he has been criticised for selling about 5m euros worth of shares after having forced through the minimum wage for postal workers - something which bolstered the company's share price.
He has other business roles, including leading Deutsche Telekom's supervisory board.