Prosecutors said they were investigating Dmitry Kovtun on suspicion of improper handling of radioactive material.
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.
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 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.
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 being treated as a suspect in an investigation of possible improper handling of radioactive material, even though it could not be determined whether polonium was inside his body or whether he was carrying it separately.
Any connection with Litvinenko's death was a matter for British police to clarify, he said.
Kirchner from the radiation agency said it was possible for Kovtun could already have been poisoned and that he left behind traces through body fluids such as sweat.
Meanwhile, the widow of Alexander Litvinenko says she blames the Kremlin for her husband's death. Marina Litvinenko says she doesn't want to help Russian authorities with their investigation because she doesn't believe they'll "tell the truth."
Litvinenko says she doesn't think Russia's President Vladimir Putin is personally responsible for killing her husband but believes it could have been Russian authorities.
Speaking to The Sunday Times on Saturday, Marina Litvinenko recalled the events of her last days with her husband, whom she refers to affectionately as Sasha.
Litvinenko first told his wife that he was feeling unwell on November 1, and after two nights of feeling sick at home, Litvinenko was admitted to hospital.
At first, Marina said medical staff told her that her husband had an infection or bug of some kind, but she said his symptoms looked "absolutely unusual."
Litvinenko told his wife very early on he felt like someone who had been poisoned — a claim she dismissed outright.
But her husband told her, "'Marina, I feel like people who was poisoned with chemical weapon,' he recognised this," Marina said because Litvinenko has studied the symptoms in military school.
Regardless of his suspicions, she said that her husband was still not sure what was wrong with him.
"One side he saw it was unusual, but he tried to believe it's not poisoning," she said.
Marina said at first medical staff did not tell her he was being moved to intensive care due to his worsening condition.
It was "in case if he needs maybe some transplantation (transplant)," Marina said. "When he moved from sixteen floor to third floor intensive care, (I was told) it was again not because he (had) became more ill. But I saw (his condition) it was (worse) every day, every day he became worse."
There was one vivid moment, Marina said that she felt "suddenly" that she could lose her husband.
It was "so sharp," she said through tears.
Litvinenko, 43, died in London on November 23 after blaming President Vladimir Putin for his poisoning in a deathbed message — an accusation the Kremlin has vehemently denied.
British police, meantime, confirm that two of their detectives on the case are among a number of people who've tested positive for traces of the radioactive substance that killed Litvinenko.