German investigators have linked an unsolved series of murders of eight foreign-born food vendors and shopowners, nicknamed the doner murders to a neo-Nazi terrorist cell, the federal prosecutors' office said on Friday.

They also linked the same neo-Nazi group to the killing of a policewoman in Heilbronn in southwest Germany in 2007.

Police searching a burnt-out motorhome in the eastern German town of Zwickau found the bodies of two men suspected of having taken part in the Heilbronn attack on two police officers, when the female agent was killed and her male colleague injured.

They also found, in the charred motorhome and a house nearby which had also been set ablaze, the two handguns used in the police woman's murder and the so-called doner murders that took place between 2000 and 2006. They also discovered neo-Nazi videos.

Federal Prosecutor Rainer Griesbaum said the deaths of the seven men of Turkish origin and one Greek, who owned or worked at shops all around the country which included doner kebab stands, would now be treated as suspected acts of terrorism.

There is sufficient evidence to attribute the murders to an extreme right-wing group, said a statement from the Federal Prosecutors' Office.

The evidence included DVDs prepared for sending to news agencies and Muslim cultural centres, containing propaganda for a group called the National Socialist Underground, including details suggesting inside knowledge of the eight murders.

Police identified the two men found dead in the caravan as Uwe B and Uwe M and said they had also arrested a woman identified as Beate Z who was described as their companion.

All of them were known to have belonged to extreme right-wing groups.

The woman faces charges of murder, attempted murder, arson and belonging to a terrorist organisation and police did not rule out connecting more neo-Nazis to the case.

The head of the German police trade union, Bernhard Witthaut, said the evidence so far suggested a right-wing extremist terror cell has left an atrocious trail of blood.

(Reporting by Sabine Siebold and Thomas Krumenacker; Writing by Stephen Brown; Editing by Sophie Hares)