A two-year investigation into the skulls found at the grave of German poet Freidrich Schiller's shows neither to be a match.

It has prolonged a 180-year-old mystery over the celebrated German poet's remains.

A team of international experts came to their surprise conclusion after comparing DNA samples from the two skulls in question to material from the graves of the poet's relatives.

The President of the Klassik Stiftung in Weimar, Hellmut Seeman, believes the lost skull is probably in a private collection.

Among the suspects is a physician from Weimar, Ludwig Friedrich, Mr Seeman said.

German anthropologists from Berlin and Freiburg had initiated the project and earlier this year they exhumed the graves of several of Schiller's family members in southern Germany.

The puzzle over the skull began in 1826, 21 years after Schiller died when the mayor of Weimar had 23 skulls dug up from a mass grave in which the poet was buried.

The whereabouts of the dramatist's skull is now a mystery.

Schiller, who lived between 1759 and 1805, wrote plays which are still performed regularly both in Germany and abroad. His poems including the Ode to Joy which Beethoven set to music in his Ninth Symphony.

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