German authorities said Sunday they have found traces of the rare radioactive substance polonium-210 at an apartment visited by a contact of poisoned ex-spy Alexander Litvinenko before they met in London.

Prosecutors said they were investigating Dmitry Kovtun on initial suspicion of improper handling of radioactive material, but said any connection with Litvinenko's death was for British police to clarify.

Investigators said the Russian businessman visited his ex-wife's Hamburg apartment the night before heading to London, where he met Litvinenko on Nov. 1 — the day the former spy is believed to have fallen ill.

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Litvinenko was killed by polonium-210. Gerald Kirchner of the German Federal Radiation Protection agency said at a news conference that tests on traces of radiation at the apartment "clearly show that it is polonium-210."

Kovtun arrived in Hamburg from Moscow on Oct. 28 on an Aeroflot flight, officials said.

Radiation was found on a couch where Kovtun is believed to have slept in his ex-wife's apartment, on a document he brought to Hamburg immigration authorities and in the passenger seat of the BMW car that picked him up from Hamburg airport, police said.

Traces of radiation were also found in Kovtun's ex-mother-in-law's residence outside Hamburg.

Kovtun is reportedly being treated in Moscow for symptoms of radiation poisoning. On Saturday, the plane aboard which he flew to London from Hamburg on Nov. 1 tested negative for polonium-210.

Prosecutor Martin Koehnke said Kovtun was not initially treated as a suspect because of the possibility that the polonium was inside his body.

Subsequently, Koehnke said, an investigation against him on suspicion of improper handling of radioactive material was begun "because at least at the moment, at this stage of the investigation, we have sufficient initial cause to believe that he brought the polonium traces to Hamburg outside his body, or that these traces are the result of contact with polonium 210."

Kirchner from the radiation agency said it was possible Kovtun could already have been poisoned and that he left behind traces through body fluids such as sweat.

Police said Saturday that traces of alpha radiation had been found at the homes in and near Hamburg of the ex-wife and the former mother-in-law of Kovtun.

The Russian businessman met Litvinenko in London on Nov. 1, the day Litvinenko fell ill. Litvinenko — an ex-Russian agent who was a fierce Kremlin critic — died Nov. 23 of poisoning from polonium-210 after making deathbed accusations blaming Russian President Vladimir Putin for the poisoning.

The Kremlin has vehemently denied involvement.

German police started looking into the case when it emerged that Kovtun flew to London from Hamburg on Nov. 1 — the day he and two other Russians met Litvinenko at London's Millennium Hotel. Kovtun reportedly is being treated in Moscow for radiation poisoning.

On Saturday, the plane aboard which Kovtun flew to London tested negative for polonium-210.

Investigators worked through the night on the case, Hamburg police spokesman Andreas Schoepflin said.

Police said they found no traces of radiation at a Hamburg apartment used by Kovtun himself.

However, they did find traces of radiation at his ex-wife's apartment — in the same building as his — and at her mother's house just outside Hamburg, and have launched further tests to pinpoint whether it was polonium-210. The two women have been questioned.

"Small traces of radioactive substances were detected, and there is a high degree of probability that this is polonium," Germany's Federal Office for Radiation Protection said in a statement Sunday.

Experts also were carrying out tests on a car at the former mother-in-law's home, police said Sunday. They did not say in whose name it was registered.

Police have said it is unclear whether Kovtun returned to Hamburg after meeting Litvinenko. They also have said their search was "purely protective" and not part of any investigation against Kovtun.

Andrei Lugovoi, one of the Russians at the Nov. 1 meeting with Litvinenko, has denied that the men were involved in the ex-spy's death.

Lugovoi, undergoing tests in Moscow for radioactive poisoning, said he was in stable condition, a Russian news agency reported Sunday.

Scotland Yard officers from Britain are in Moscow to question Lugovoi, but the interrogation has been postponed several times because of his health.

Lugovoi told the RIA Novosti news agency he wouldn't face questioning Sunday. He said his condition was "stable" and results of his medical checks would be available by the end of the week.

Doctors examining Lugovoi for any signs of illness due to possible exposure to radiation have refused to answer questions about his condition.

Lugovoi said Sunday that Kovtun also was in "satisfactory" condition. "He's not in a coma" as reported, Lugovoi told the RIA Novosti.

The Interfax news agency had reported Thursday that Kovtun slipped into a coma after being questioned by Russian investigators and Scotland Yard detectives. Lugovoi has dismissed the report as "lies."

Russian prosecutors have filed a criminal case for the murder of Litvinenko and attempted murder of Kovtun, and the chief prosecutor's office said Saturday it was going to send investigators to London as part of the probe.

The move to open a criminal probe in Russia would allow suspects to be prosecuted in Russia. Officials previously have said that Russia would not extradite any suspects in Litvinenko's killing.

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