HAMBURG, Germany (AP) – Traces of the rare radioactive substance polonium-210 were found at a German apartment visited by a contact of fatally poisoned ex-spy Alexander Litvinenko – before the two men met in London, authorities said Sunday.

The polonium traces were found on a couch where Russian businessman Dmitry Kovtun is believed to have slept at his ex-wife’s Hamburg apartment the night before he headed to London for a meeting with Litvinenko last month, German investigators said.

Tests on traces of radiation at the apartment “clearly show that it is polonium-210,” Gerald Kirchner of the Federal Radiation Protection agency said at a news conference.

Investigators said Kovtun flew to Hamburg from Moscow with Aeroflot on Oct. 28 and departed for London on Nov. 1. That is the day when Kovtun and at least one other Russian met with Litvinenko at London’s Millennium Hotel – and when Litvinenko is believed to have fallen ill.

Traces of radiation also were found in the passenger seat of a car that picked Kovtun up from the Hamburg airport, on a document Kovtun brought to Hamburg immigration authorities and at the home of Kovtun’s ex-mother-in-law outside Hamburg – all from before the Nov. 1 meeting.

German prosecutors did not say whether they suspect Kovtun might have been involved in Litvinenko’s death. But they said they were investigating him on suspicion he may have improperly handled radioactive material.

“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,” prosecutor Martin Koehnke said.

Officials said that any connection between Kovtun and Litvinenko’s death would have to be investigated by British police. British police are treating his death as a murder.

“We still believe that both variants are possible: that he may be a victim, but also that he may have been involved, at least in procuring the polonium,” Koehnke said.

Litvinenko, an ex-Russian agent who was a fierce Kremlin critic, died Nov. 23 after blaming Russian President Vladimir Putin for the poisoning. The Kremlin has vehemently denied involvement.

Kovtun reportedly is being treated in Moscow for radiation poisoning. Russian authorities have opened a criminal investigation into his poisoning, calling it attempted murder.

Kirchner, the radiation agency official, said it was possible Kovtun could already have been poisoned when he arrived in Hamburg and left behind traces through body fluids such as sweat.

On Saturday, the German plane aboard which Kovtun flew from Hamburg to London tested negative for traces of polonium-210. Investigators raised the possibility that may be because the plane had been cleaned thoroughly.

A security officer for Moscow’s Sheremetyevo airport said all Aeroflot planes, including the one which flew to Hamburg on Oct. 28, had been checked for radiation and tested negative. The officer spoke on condition of anonymity because he wasn’t authorized to speak to the media.

Litvinenko met at the Millennium Hotel in London’s Mayfair neighborhood with Kovtun and former Soviet agent Andrei Lugovoi. Another man, security firm head Vyacheslav Sokolenko, has said he was at the hotel but did not participate in the meeting.

Lugovoi has denied that the men were involved in the ex-spy’s death.

Meanwhile, Litvinenko’s widow said in interviews published Sunday that her late husband’s criticism of the Kremlin had antagonized his former secret service colleagues, and contended that Putin had created an atmosphere that “makes it possible to kill a British person on British soil.”

In her first interviews, Marina Litvinenko said she believed Russian authorities were behind the poisoning of her husband, who sought asylum in Britain in 2000 and obtained citizenship this year. Marina Litvinenko told Sky News in an English-language interview that her husband “openly went out from system and accused the system of killing people, of kidnap.”

Marina Litvinenko has placed her faith in British investigators but said she does not intend to cooperate with Russian authorities, who plan to come to London to probe her husband’s death.

“In Russia, it doesn’t matter how many people are killed,” she said, adding that the life of “only one person can still be very important in England.”

Also on Sunday, Litvinenko’s friend Alex Goldfarb accused Russian authorities of trying to obstruct the British probe by preventing Kovtun and Lugovoi from being questioned.

Lugovoi was supposed to testify after a team of Scotland Yard officers arrived in Moscow on Tuesday. But the interrogation has been postponed several times, although Lugovoi himself has said he is eager to answer questions.

“It’s a clumsy effort to cover up the trace, to prevent British investigators from meeting with two key witnesses,” Goldfarb told The Associated Press.

He added that Lugovoi and Kovtun could be in danger as the authorities “could try to remove them later.”

“Another crime is unfolding before our eyes – the removal of two key witnesses: Lugovoi and Kovtun,” he told the AP.

Lugovoi, who is being checked in Russia for radioactive poisoning, said Sunday his condition was “stable” and results of his medical checks would be available by the end of the week.

Lugovoi said Kovtun also was in a “satisfactory” condition. “He’s not in a coma,” Lugovoi told the RIA Novosti, denying a report by the Interfax news agency on Thursday.