Prosecutors in Germany have searched the home of a defence ministry employee suspected of spying, in the second such case in a week.
Residential and office areas were searched in Berlin on Wednesday morning, the federal prosecutor in Karlsruhe said.
An intelligence agency employee was arrested this month for passing secret documents to US intelligence.
The US promised on Monday to work to resolve the problem.
There were no reports of a new arrest on Wednesday.
The US has not denied allegations that the intelligence agency employee arrested earlier this month was passing secret documents to the US National Security Agency (NSA).
German media say the latest investigation is more serious than the intelligence agency (BND) case.
The German defence ministry said an investigation was under way, without giving details.
Timeline: Germany spy scandal
October 2013: German Chancellor Angela Merkel calls US President Barack Obama after revelations that the US may have spied on her mobile phone
June 2014: Germany announces an investigation into the claims
4 July 2014: A suspected US spy said to be working for the German intelligence agency is arrested
9 July 2014: A second case emerges as searches are carried out at the defence ministry
Prosecutors did not say which foreign state the new case concerned but German media described the inquiry as a new "US spy" case.
The US embassy in Berlin has not commented on the allegation, but ambassador John Emerson had talks at the German foreign ministry on Wednesday with State Secretary Stephan Steinlein. He last visited the ministry only five days ago.
German Chancellor Angela Merkel said on Monday that if confirmed, the earlier case would be a "clear contradiction of what I consider to be trusting co-operation" with the US.
The two countries, the biggest members of the Nato alliance, have been close allies for decades but relations were strained last year when it was revealed that the NSA had been monitoring Mrs Merkel's mobile phone calls.
US whistleblower Edward Snowden, a former NSA contractor who remains a fugitive in Russia, revealed the scale of NSA surveillance.
The BND employee arrested earlier is thought to have been involved in a German parliamentary investigation into the activities of foreign intelligence agencies, including the NSA.
Chancellor Merkel has proposed establishing a European communications network to avoid emails and other data automatically passing through the US.