German authorities are investigating a second spy case reportedly involving the U.S., 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.

The daily Sueddeutsche Zeitung reported, without naming sources, that the man being investigated worked in “the military field” and is suspected of spying for the United States.

A 31-year-old German intelligence employee was arrested last week on suspicion of spying for foreign powers since 2012. German media have reported he spied for the United States and offered his services to Russia.

The case has frayed relations between Berlin and Washington, which were already strained by reports last year that the National Security Agency spied on Chancellor Angela Merkel’s cellphone.

The U.S. ambassador to Berlin was summoned to the Foreign Ministry on Friday after news of that case broke. He was asked to help clarify the case.

Ambassador John B. Emerson was at the ministry again on Wednesday for a meeting with a senior official, said a German official who spoke on condition of anonymity because he wasn’t authorized to discuss the case publicly. It wasn’t immediately clear whether Emerson was summoned and whether the discussion involved the new case.

(AP)