The Tribune de Genève said on Thursday that the bank had filed a complaint with cantonal justice authorities against one of its former longtime Spanish employees over the disappearance of the funds.
Vontobel discreetly filed the complaint with the public prosecutor on December 24th, the Tribune reported.
The former employee, who had apparently worked as an independent asset manager for two years after leaving Vontobel, is no longer in Switzerland and believed to be in Spain, the newspaper said.
Spanish media have reported on the case, saying large sums of money were placed with the Swiss bank by rich Catalonian families seeking to evade taxes.
Last year the families decided to take advantage of a tax amnesty offered by the Spanish government.
But when they moved to repatriate their funds they discovered that most of the money had disappeared, according to the Tribune de Genève.
It cited a report from the Spanish website Economiadigital which said that the families affected discovered only 20 to 30 percent of their capital remained.
The asset manager was reportedly on good terms with his clients, issuing bank certificates showing annual returns in the order of five to 10 percent.
The Tribune said that it was only later that they realized “the documents were probably false”.
It is not clear whether the money was lost through high-risk investments or misappropriated.
Geneva prosecutor Dario Zanni told the Tribune de Genève that a complaint had been filed by Vontobel “against a Spanish asset manager currently on the run”.
A spokesman from the bank declined to comment on the case because of the ongoing investigation.