Traces of radiation were found in two buildings linked to Dmitry Kovtun, a Russian businessman who met Litvinenko in London on the day he fell ill last month.
Kovtun is currently in hospital in Moscow, but his state of health remains unclear.
Police said they found traces of radiation in the Hamburg apartment of Kovtun's German ex-wife. They also said an initial scan had shown contamination at the home of his ex-wife's mother in Haselau, west of Hamburg.
A Hamburg police spokeswoman, Ulrike Sweden, said the "ex-wife of the 41-year-old [Kovtun] lived in that apartment along with her two children. The woman is with us and has been undergoing medical tests and a [police] interrogation."
Litvinenko was buried last week Specialists found no trace of polonium-210, the rare radioactive substance that killed Litvinenko, at Kotvun's own apartment in the Hamburg district of Ottensen.
"Four apartments in this building were investigated, among them the apartment of the 41-year-old [Dmitry Kovtun] in which nothing was detected," Sweden said. "However, in an apartment on the first floor, contamination was found in two places. Specialists are now carrying out a so-called fine scan of the building, a very complex investigation which requires the house to be empty. During the investigation there can't be people walking around in the hallways."
Kovtun traveled to London from Germany on November 1 to meet Litvinenko, he told the stern.de website in an interview conducted before he was taken to hospital.
German police say they began their probe after learning that Kovtun had traveled to London.
Litvinenko was buried last week (epa)
Airlines Germanwings said today that radiation experts are now checking the plane that Kovtun flew on to London. The A-319 aircraft was grounded today at Cologne-Bonn airport, but no test results are yet available.
Some reports say Kovtun is in critical condition, but other reports say that diagnosis is wrong.
Police said that they do net yet consider Kovtun, his ex-wife, or her mother as suspects in the probe.
Few Risks For Public
Spokeswoman Sweden said there was little radiation danger to the general public: "Polonium is a so called weak radiator, which means that the substance only releases its effect within a radius of about four centimeters. It's only dangerous when consumed or when it gets in contact with open wounds. So people passing this building or those who only saw Mr. Kovtun will not be in danger."
British detectives working in Moscow as part of their investigation into the poisoning of Litvinenko, a British citizen, have already questioned Kovtun with Russian investigators.
Kovtun met with Livinenko in London on November 1 together with his business partner Andrei Lugovoi to discuss a business deal. He has denied any part in Litvinenko's poisoning and offered to help police. He told stern.de that he would be happy to give police details of the deal if they questioned him.
Interfax news agency reported on December 8 that Kovtun's business partner Lugovoi had damage to vital organs consistent with exposure to dangerous levels of radiation. British investigators are hoping to question Lugovoi as well.
A Kremlin critic, Litvinenko died on November 23. He was buried in London on December 7.