The Leipziger Volkszeitung said Helmut Metzner, who was chief of staff to FDP leader and German Foreign Minister Guido Westerwelle, had access to at least five government documents classified as confidential.
The source of this information was said to be written statements made to SPD lawmakers by the German state secretary of the foreign ministry, Wolf-Ruthart Born.
Earlier this month, Metzner was fired after it was revealed that he was the source of leaked information on the FDP's coalition negotiations with Chancellor Angela Merkel's conservative party in October 2009.
"The staff member of the FDP's federal headquarters, who has admitted his contacts with the US embassy in Berlin, has been relieved of his duties as chief of staff for the FDP chairman by mutual agreement," the party's spokesman said in a statement.
Diplomatic cables published by Wikileaks showed that an FDP "mole" at the closed-doors talks had given regular updates to a US embassy contact.
In an interview with news magazine Focus, Metzner denied that he had betrayed German state secrets, saying he had only distributed information that was already publicly available.
"At no time did I deliver or offer documents with confidential content to embassy officials," he said.
Meanwhile, members of the political opposition took aim at Metzner's former boss, Guido Westerwelle, who as FDP leader is also vice-chancellor.
The centre-left Social Democrats' (SPD) chief whip, Thomas Oppermann, said that Westerwelle, owing to the "obvious overload of the double office," should choose between being foreign minister and head of the Free Democrats.
He also questioned Westerwelle's "political responsibility," claiming that the extent and nature of Metzner's information-feeding would not be investigated "because it's not in the interest of the FDP," and because of an "apparent commingling of party and government interests."