Police in Germany say four people have been contaminated with polonium-210, the substance used to poison the former KGB agent Alexander Litvinenko.
Homes in and near Hamburg have been tested
German officials have confirmed that Dmitry Kovtun's ex-wife, her partner and two children have been contaminated with the radioactive substance.
Mr Kovtun met Mr Litvinenko in a London hotel the day the former spy fell ill.
Meanwhile, a British detective has arrived in Hamburg to be briefed on the investigation by German police.
And a second contact is thought to have met British police in Moscow.
It has been reported that British detectives and officials from the Russian Prosecutor General's office interviewed ex-KGB bodyguard Andrei Lugovoi, who is currently in hospital.
A car, understood to be used by Mr Kovtun, has tested positive for the radioactive substance polonium-210.
Traces were also discovered in his ex-wife's Hamburg flat and the home of Mr Kovtun's former mother-in-law in the town of Haselau, west of Hamburg
German authorities say Mr Kovtun arrived in Hamburg on 28 October, and stayed at his ex-wife's flat before flying to London.
He met Mr Litvinenko at a hotel in London on 1 November.
Hamburg police say it is not yet clear whether Mr Kovtun is a victim or a suspect in the case.
The former KGB bodyguard is understood to be having treatment for radiation poisoning at a hospital in Moscow.
Both the UK and Russia are investigating the ex-spy's death
Meanwhile, friends of Mr Litvinenko have said they are unlikely to help Moscow's inquiry into Mr Litvinenko's death.
Alex Goldfarb said they did not "trust" a Russian investigation and would help only if they were given assurances by British authorities about their safety.
His comments came as it emerged that Russian investigators are to travel to London to question witnesses.
Mr Litvinenko's widow earlier said she would not assist the Russian inquiry.
Mr Litvinenko, 43, died in a London hospital on 23 November - it is suspected he was poisoned by polonium-210.
Friends of Mr Litvinenko believe he was poisoned because of his criticisms of the Russian government since defecting to the UK, but the Kremlin has dismissed suggestions it was involved in any way.
Russia's foreign intelligence service has also issued a statement denying any involvement.
THE POLONIUM TRAIL
Emirates Stadium: Traces of polonium-210 found at Arsenal's ground, where ex-KGB man Vyacheslav Sokolenko attended a match during his stay at the Millennium Hotel2
University College Hospital, where Litvinenko died on 23 November. Mario Scaramella, who met Litvinenko at sushi bar, also examined there3
Central London: Litvinenko met contacts at Millennium Hotel and Itsu sushi bar on 1 Nov - polonium-210 traces found in both places4
Heathrow Airport: Polonium-210 traces found in three aircraft frequently used in flights between London and Moscow, since given all-clear
Traces of polonium-210 found on a Finnish plane and at British embassy, visited by Andrei Lugovoi, one of those who met Litvinenko in London
Another contact Litvinenko met at Millennium Hotel - Dmitry Kovtun - treated for radiation poisoning
More polonium-210 traces found in flat of Kovtun's ex-wife, where he had stayed before flying to London, in a car he used and at his mother-in-law's home
She, her new partner and her two children are contaminated