Two containers with around 14,000 documents inside were confiscated at the end of May, with some coming from a branch of private bank Coutts, a subsidiary of the Royal Bank of Scotland, in the Cayman Islands, newspaper Welt am Sonntag said in an advance copy of an article due to be published on Sunday.
News magazine Focus also reported that customs authorities confiscated a container of Coutts documents from the Cayman Islands, which is frequently labelled as a tax haven by critics, on May 20. Finance experts had since then been evaluating the material, searching for evidence of tax evasion, Focus said.
A spokesperson for Coutts told Reuters: "We are not aware of any investigation into our Trust Company or its papers and we are working with the authorities to allow these papers to continue on their way."
In 2012, authorities in North-Rhine Westphalia investigated Coutts after the federal state bought a CD containing details of German customers of Coutts, Welt am Sonntag said.
A spokeswoman for the German finance ministry said the public prosecutors' office in Duesseldorf was the responsible authority, adding that this case was linked to investigations already being carried out in North-Rhine Westphalia.
She said she could not give further information on ongoing investigations.
The public prosecutors' office in Duesseldorf could not be immediately reached for comment by Reuters.
(Reporting by Michelle Martin; Editing by Larry King)