As experts continued to check traces of radiation found in and around Hamburg, Germany, a British police officer arrived in the city, where investigations have centered on Russian businessman Dmitry Kovtun.
Experts on Sunday said they found traces of polonium-210 — the substance that killed Litvinenko — at a visited by Kovtun the night before he departed for London Nov. 1 and met Litvinenko.
German prosecutors are investigating Kovtun on suspicion that he may have improperly handled radioactive material, but have left open whether he might have been involved in Litvinenko's poisoning.
On Monday, a British police official arrived in Hamburg and was to be briefed on the German investigation, Hamburg police said.
Investigators said Kovtun flew to Hamburg from Moscow 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.
After confirming traces of polonium-210 on the couch at his ex-wife's apartment, where Kovtun is believed to have slept on the night of Oct. 31, German investigators checked indications of radiation found at other sites he visited.
Tests on traces found in the passenger seat of the BMW car that picked up Kovtun from the Hamburg airport showed that "it is definitely polonium," police spokesman Andreas Schoepflin said.
Radioactive traces also have been found on a document Kovtun brought to Hamburg immigration authorities; and at the home of Kovtun's ex-mother-in-law outside Hamburg — again from before the Nov. 1 meeting.
Russian news agencies reported that British police in Russia and their Russian counterparts questioned another Russian at the Nov. 1 meeting, Andrei Lugovoi, at a Moscow hospital that specializes in treating radiation cases on Monday. Kovtun is also believed to be at the hospital.
Lugovoi told the ITAR-Tass news agency that the questioning lasted three hours.
"I gave testimony exclusively as a witness. I was officially informed of that before the interrogation," ITAR-Tass quoted him as saying. "They made no charges against me."
Lugovoi said the results of his medical tests would be known later this week, but added that he was "unlikely" to make them public.
Litvinenko — an ex-Russian agent who was a fierce Kremlin critic — died Nov. 23 of poisoning from polonium-210 after blaming Russian President Vladimir Putin, also a former intelligence officer, for the poisoning.
The Kremlin has vehemently denied involvement.
German officials say they believe Kovtun already was contaminated with polonium-210 when he arrived in Hamburg — but how that happened is unclear.
"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," prosecutor Martin Koehnke said on Sunday.
Kovtun reportedly is being treated in Moscow for radiation poisoning. Russian authorities, calling it attempted murder, have opened a criminal investigation into his poisoning.
Litvinenko met at the Millennium Hotel in London's Mayfair neighborhood with Kovtun and Lugovoi. Another man, security firm head Vyacheslav Sokolenko, has said he was at the hotel but did not participate in the meeting.
Lugovoi said Sunday that Kovtun was in "satisfactory" condition. "He's not in a coma" as has been reported previously, Lugovoi told Russia's RIA Novosti news agency.