A spokeswoman for the German Interior Ministry said on Friday evening that the scare appeared to have been a false alarm, with tests failing to confirm an initial suspicion that the letter contained a highly explosive substance.

The correspondence intercepted on Friday had been addressed directly to the president himself, according to the president's office. The DPA news agency reported that the alarm had been raised after the letter was scanned by X-ray.

The package was detonated by experts in parkland belonging to the presidential residence, the Schloss Bellevue palace in Berlin.

Germany's Federal Criminal Police Office (BKA) immediately launched an investigation. The president's office said that Joachim Gauck was not at the palace at the time the letter was intercepted in a routine check and that no staff were endangered.

A sample of a substance inside the package was taken by police for testing, although no further details about the contents of the envelope were immediately made public.

Clues about the source of the letter and the possible motive for it being sent were not immediately apparent.

According to the website of German daily newspaper "Die Welt," the letter was thought to contain the highly explosive compound hexamethylene triperoxide diamine (HMTD), in the form of a powder.

A letter bomb to Chancellor Angela Merkel was intercepted in 2010, one of a number sent to high profile figures. A Greek anarchist group claimed responsibility for that incident.

rc/msh (AFP, dpa, Reuters)