A suspected double agent under arrest in Germany appeared to have been working for the CIA for two years, investigators said after allegedly uncovering an American spying device on his computer.
"All the evidence suggests that he was working for the Americans," an unnamed senior security official told the Frankfurter Allgemeine Zeitung.
The country's top security official demanded a full American response to the German investigations.
"I expect everyone now to assist quickly in clearing up the accusations - and quick and clear statements, from the US too," Thomas de Maiziere, the Interior Minister said.
The Foreign Ministry summoned the American ambassador on Saturday even though German authorities have confirmed only that a 31-year-old man is under arrest. The man, who is severely disabled, worked in the Foreign Relations department of Germany's BND intelligence service, where he had access to classified documents and information about agents abroad.
For the last two years, he has been sending documents to the Americans electronically once a week, according to Bild newspaper.
Investigators reportedly found an encrypted communication program hidden on his computer.
The hidden program was disguised as a weather app, and activated by searching for a weather forecast for New York.
The arrested man was able to carry on most communications electronically, but met an American contact in Austria on two or three occasions, when he allegedly received payments of 25,000 ($38,900) in cash. Reports revealed that the man was caught after an anonymous email he sent offering to sell classified information to the Russians was intercepted.
News of the arrest has been greeted with fury in a Germany still reeling from the revelation that the NSA spied on Chancellor Angela Merkel's phone calls.
Merkel has yet to comment publicly on the case.