German authorities are investigating a second spy case reportedly involving the United States, a week after the arrest of a German intelligence employee cast a new shadow over relations between the two countries.

Federal prosecutors said Wednesday that police raided properties in the Berlin area on “initial suspicion of activity for an intelligence agency.” They did not elaborate or specify what intelligence agency was involved, but said they had not made an arrest.

“We have investigations in two cases of suspected espionage, a very serious suspicion,” government spokesman Steffen Seibert later told reporters in Berlin. He declined to provide further details, citing the ongoing investigations.

The daily Sueddeutsche Zeitung reported, without naming sources, that the man being investigated worked at Germany’s Defense Ministry and is suspected of spying for the United States.

News website Spiegel Online reported, also without naming sources, that the man worked in a department dealing with international security policy and had aroused the suspicion of Germany’s military counter-intelligence agency because of his close contacts to alleged U.S. spies.

Defense Ministry spokesman Lt. Col. Uwe Roth declined to confirm the reports, but said the case fell “into the ministry’s area of responsibility” and that Defense Minister Ursula von der Leyen had been informed.

State Department officials traveling with U.S. Secretary of State John Kerry in Beijing had no immediate comment. The White House also decided to comment.

Last week, a 31-year-old German intelligence employee was arrested on suspicion of spying for foreign powers since 2012. German media have reported that he spied for the United States and came to authorities’ attention when he recently offered his services to Russian officials in Germany by email.

The case has frayed relations between Berlin and Washington, which were already strained by reports last year that the National Security Agency had targeted Chancellor Angela Merkel’s cellphone and conducted mass surveillance of Internet traffic in Germany. Those allegations have resulted in a criminal investigation and the creation of a parliamentary panel tasked with probing the NSA’s activities in Germany.

The U.S. ambassador to Berlin was summoned to the Foreign Ministry on Friday after news of the first case broke.

Lora Anne Viola, an assistant professor in American foreign policy at Berlin’s John F. Kennedy Institute, said the spy cases appeared to mark a low in U.S.-German relations.

“I think it’s going to be difficult to repair this with words alone,” she said, adding that it was in Washington’s interest to reach out to Germany.

“Without partners, without Germany, it’s very difficult for the U.S. to act on the foreign stage,” Viola said. She cited the case of Ukraine, where Germany has played a key role reaching out to Russia while also rallying European countries around the idea of sanctions.