Germany summoned the U.S. ambassador on Friday after the arrest of a man who had reportedly spied for the United States, fueling tensions that had already intensified over alleged U.S. eavesdropping.
U.S. Ambassador John Emerson was called "in connection with an investigation by the federal prosecutor," the German Foreign Ministry said in a statement.
The German federal prosecutor said a 31-year-old German had been arrested on Wednesday on suspicion of working for foreign intelligence agencies, according to news reports.
Those agencies weren’t identified and the man's links to U.S. intelligence haven't been conclusively established.
The worker initially raised suspicions because he apparently tried to contact Russian spies, but then told investigators he had been passing information to U.S. intelligence, the Wall Street Journal reported.
German government spokesman Steffen Seibert told reporters that there was enough concern that Chancellor Angela Merkel been personally informed of the arrest.
"This is a very serious development," a spokesman for Merkel said. "The government will now await the conclusions of the federal prosecutor and federal criminal police's investigation."
German newspapers reported that the man was suspected of passing on information about a German parliamentary committee investigating the activities of U.S. and other intelligence agencies in Germany.
He claimed to have worked with U.S. intelligence since 2012, they reported.
Reports that the National Security Agency spied on Germans, including Merkel's cellphone, have created friction between Berlin and Washington since the reports based on documents leaked by former NSA contractor Edward Snowden came out last year.