"We can rule out any danger for these four people," Kirchner said.


Police had said on Monday that the four had shown signs of contamination after Kovtun had stayed at their home.


Kovtun is one of three Russians who met Litvinenko at a London hotel on November 1, the day the former FSB agent and critic of Vladimir Putin, the Russian president, became fatally ill.


Investigation continues


He is being investigated for bringing polonium 210, the radioactive substance that killed Litvinenko, into Germany in late October when he passed through on his way from Moscow to London.


Traces of polonium 210 were found in the apartment of Kovtun's ex-wife, her mother's home and in a car he used and an immigration office he visited.


The discovery has widened the British-led investigation into Litvinenko's death. A Scotland Yard detective flew to Hamburg on Monday to meet German police.


Meanwhile, British detectives in Moscow questioned Andrei Lugovoi, a business associate of Kovtun, who was also at the meeting with Litvinenko at the Millennium Hotel in London.


British police are reportedly increasingly convinced that the poisoning took place at the hotel.


Litvinenko's death has prompted allegations that the Russian authorities are using dirty tricks to silence their critics.


Litvinenko claimed from his deathbed that he had been a target of Putin.


Condoleezza Rice, the US secretary of state, urged Moscow on Tuesday to co-operate fully with the investigation into his death.


Russia has strongly denied having any role in the death.