The four had contact in Germany with Russian businessman Dmitri Kovtun, who spent four days in Hamburg in late October before flying to London, where he and two other Russian men met at a hotel with Litvinenko on Nov. 1. Litvinenko fell ill later that day from radiation poisoning and died several weeks later.

German prosecutors have started a criminal investigation of Kovtun, who is reported to be in a Moscow hospital. They suspected he may have illegally handled polonium and could also have left traces of it after being contaminated himself.

Police confirmed Monday that evidence of polonium was found on a seat in a BMW that picked up Kovtun at the Hamburg airport on Oct. 28. There were also traces in a second car he used.

While in Hamburg, Kovtun slept for two nights in the apartment of his former wife. Police said they detected radioactive traces on a child's car seat, and in two bedrooms, suggesting that the ex-wife, her two children and her boyfriend had been contaminated.

The four are not sick, according to Ulrike Sweden, a ??? police spokeswoman, and it will take a few days of tests to determine whether they ingested the substance. She said there was no public health danger.

Andrei Lugovoi, another key figure in the case, was questioned in Moscow on Monday by Russian and British detectives, Itar-Tass news agency reported.

The British investigators arrived in Moscow on Dec. 4. They and their Russian colleagues interviewed Kovtun last week.

Scotland Yard detectives and Russian prosecutors visited a Moscow hospital that specializes in treating radiation cases on Monday and interrogated Andrei Lugovoi on the afternoon, the Russian news agency said.

Lugovoi told Itar-Tass 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 after questioning.

The British investigators arrived in Moscow on Dec. 4. They and their Russian colleagues interviewed Russian businessman Dmitry Kovtun, another key figure in the case, last week.

German investigators have been focusing on Kovtun since traces of polonium-210, the radioactive element that killed Litvinenko, were found in and around Hamburg last week.

Kovtun visited the city, where his ex-wife lives, on the eve of his Nov. 1 trip to London, where he and Lugovoi met with Litvinenko. Litvinenko said he fell ill that day and he died three weeks later.

Kovtun has also been diagnosed with radiation sickness.

German authorities discovered radioactive traces linked to Kovtun that he left while visiting the city days before his Nov. 1 meeting with Litvinenko, who died last month in London following radiation poisoning. The probe expands a case involving former agents and Russian-émigré businessmen that has strained U.K.-Russian relations and drawn scrutiny to President Vladimir Putin.

Hamburg Police spokesman Andreas Schoepflin said it was unclear whether Kovtun, who is also currently being treated for radiation poisoning in Moscow, knowingly handled the material or was one of the latest victims of alpha-ray contamination. A U.K. investigator from London's Metropolitan Police arrived today to assist the probe.

It's also not clear whether the other four were contaminated, which doesn't represent an imminent danger, or have ingested the substance, the police said in a statement. Ingestion could only occur as a result of "very intensive bodily contact'' with Kovtun.

Litvinenko's Death

Litvinenko, a critic of the Kremlin, died on Nov. 23 after exposure to polonium 210. Authorities have detected radiation contamination in at least 12 London locations and British police are treating the inquiry as a murder investigation. Russian authorities on Dec. 7 opened a criminal case into Litvinenko's death and what it described as the attempted murder of Kovtun.

In a statement dictated two days before his death Litvinenko -- a former officer of Russia's FSB state security agency -- blamed his illness on the government of Putin, who has denied the allegation.

Kovtun is suffering from an acute form of radiation sickness, Russia's Interfax reported on Dec. 8, without saying where it got the information. Kovtun's illness was caused by the radioactive poisoning which affected Litvinenko, Russian prosecutors said. Kovtun's cell phone was switched off.

Russian investigators aren't yet cooperating with the investigation, Hamburg police spokesman Schoepflin said.

Days in Hamburg

Kovtun, 41, who has an apartment in Hamburg, arrived on an Aeroflot flight from Moscow on Oct. 28 and spent four nights in the northern German city, the police said. Police have reconstructed his activities between the apartment of his 31- year-old ex-wife, and stops at the immigration office, restaurants and a gambling hall, according to a statement.

Traces of polonium were found in the ex-wife's apartment, another apartment in Hamburg's Pinneberg district, in the seat of the BMW that drove him from the airport and on a file that he signed at the registration office, according to the police.

Kovtun flew to London at 6:40 a.m. on Nov. 1 on a flight operated by Deutsche Lufthansa AG's Germanwings unit. There he met at the Millennium Hotel Mayfair in central London's Grosvenor Square with Litvinenko and another Russian businessman who has been treated for radiation exposure, Andrei Lugovoi.

Russian and British investigators began to question Lugovoi at a closed Moscow hospital, Interfax reported today. The newswire also reported that Russian investigators plan to fly to London this week to probe Litvinenko's death.

The German investigation into Kovtun centers on "suspected abuse of ionizing radiation and for illegal handling of radioactive material,'' Koehnke said in a statement yesterday.

Enemies at FSB

Litvinenko's death was caused by leukemia, north London coroner Andrew Reid said on Nov. 30. Leukemia, a cancer of the blood or bone marrow, can be brought on by radiation.

Litvinenko's wife Marina, 44, told the U.K.'s Mail on Sunday yesterday that Russian authorities may have been involved with her husband's death, echoing the final statement by Litvinenko directed at Putin's government. She said Litvinenko had enemies at Russia's FSB, though she said it was unlikely that Putin had ordered his assassination.

"Obviously it was not Putin himself, of course not,'' she told the newspaper. "But what Putin does around him in Russia makes it possible to kill a British person on British soil. I believe that it could have been the Russian authorities.''

To contact the reporter on this story: Patrick Donahue in Berlin at pdonahue1@bloomberg.net .

However, the non-proliferation of nuclear weapons should also be observed, said Kiriyenko, who is making a visit for an Iran-Russia Economic Commission meeting in Tehran.

"The right for use (of nuclear energy) and non-proliferation of nuclear weapons should be definitely observed. Of course it is too hard to mix the two points but a breakthrough should be found," Kiriyenko said.

The Russian official held that Iran's nuclear case should only follow the diplomatic course for a political solution, adding "Iran's nuclear case should be settled diplomatically and in this connection, Russia's stances are the same both in words and action."

"Russia will never yield to imposition and Iran's nuclear case should have only a political solution," Kiriyenko stressed.

For his part, Mottaki reiterated Iran's determination to develop a full nuclear fuel cycle, saying "we have already developed our nuclear facilities and access to relevant technology has been materialized by Iranian scientists."

"Iran aims to access nuclear fuel cycle. However, this does not interfere with our assessment of other proposals," Mottaki said, adding that Iran was still studying Russia's proposal to set up a uranium enrichment company in the Russian territory.

Mottaki, meanwhile, praised that implementation of Iran's Bushehr nuclear power plant marks an expansion of Iran-Russia relations.

"Bushehr nuclear power plant project will be on the agenda of the meeting between Kiriyenko and head of Iran's Atomic Energy Organization Gholam-Reza Aqazadeh," he added.

Iran, the world's fourth largest oil exporter, says it needs to enrich uranium as a peaceful, alternative energy source and has the right to do so under the nuclear Non-Proliferation Treaty (NPT).

However, the West has accused Iran of trying to produce nuclear weapons under a civilian cover, a charge has been denied by Tehran.

Due to Iran's resistance to suspend uranium enrichment, the European countries and the United States have been seeking a UN Security Council resolution to impose sanctions on Tehran.