Russia's top prosecutor said Tuesday that Moscow will not extradite possible Russian suspects in the poisoning of ex-agent Alexander Litvinenko to Britain, and warned that visiting British detectives would only be allowed to listen as their Russian counterparts collect testimony.

Yuri Chaika said that under Russian law, a Russian citizen who is accused of committing a crime abroad must face trial at home.

"If they want to arrest citizens of the Russian Federation, it would be impossible because of the Russian Constitution," Chaika told reporters.

Litvinenko, 43, died Nov. 23 in London. Toxicologists found polonium-210, a rare radioactive substance, in his body.

In a deathbed accusation, Litvinenko blamed President Vladimir Putin for the poisoning. The Kremlin has vehemently denied the accusations.

Chaika said that his office would fully cooperate with a team of British investigators who arrived in Moscow Monday to collect information on Litvinenko's death.

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At the same time, he said that all figures in the case whom the British investigators wanted to interrogate would be questioned by Russian prosecutors in the presence of the British officers.

"It is we who are doing all the interrogating. They only attend that, it cannot be otherwise" in Russia, Chaika said.

Chaika also confirmed that a potential central figure in the case, another former Russian agent who met with Litvinenko in London on Nov. 1 — the day Litvinenko believed he was poisoned — was in a hospital.

Chaika said the British officers could be allowed to visit Andrei Lugovoi and listen to him being interrogated by Russian prosecutors, if doctors permit that.

"He's being treated in a hospital," Chaika said. "Everything will depend on the doctors' opinion. If doctors allow a conversation with him, he will be questioned."

Lugovoi told the ITAR-Tass news agency that he was undergoing special tests for possible radiation contamination, and that the results would be ready in a few days.

He said he was prepared to answer all the British investigators' questions.

"I intend to fully satisfy their interest and am waiting for an invitation from the law enforcement organs," he was quoted as saying.

"Once I give all the necessary testimony to the law enforcement organs, I intend to publicly put an end to (speculation) about my supposed involvement in this story that has caused such a stir," he said.

Lugovoi traveled to London three times over the month before Litvinenko's death and met with Litvinenko four times, according to Russian media. He said Litvinenko had contacted him from London about a year ago with some business-related proposals, and that they had met intermittently in London since then.

Chaika refused to say how many people the British investigators wanted to produce testimony in the case, but said that the originally requested list couldn't be expanded on during the visit.

He said that former security officer Mikhail Trepashkin, who is now serving out a four-year prison sentence after being convicted of divulging state secrets, wasn't part of the British request and that British investigators would not be allowed to talk to him.

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Trepashkin has claimed in a letter from prison that he had warned Litvinenko several years ago about a government-sponsored death squad that intended to kill him and other Kremlin opponents.

Chaika dismissed Trepashkin's allegations, saying that "we aren't obliged to react to any stupidity."

A court in the Ural Mountains region where Trepashkin has been serving his sentence was set to consider the prison administration's request to put him in a higher-security barracks, Trepashkin's lawyer Yelena Liptser said.

Chaika also shrugged off questions about the possible questioning of officials from the Federal Security Service, a top KGB successor agency, which some observers have accused of involvement in Litvinenko's death.

"Why should FSB officials be questioned? Why not question everyone here?," he said.

Following other Russian officials, Chaika rejected allegations that the polonium-210 that killed Litvinenko could have been smuggled from Russia.

"It couldn't happen here," he said, adding that two Russian plants mentioned in the British media as possible sources of polonium-210 used in the murder don't produce it.

"It's all sheer nonsense," he said of the allegations.

The case has further strained already tense relations between Russia and Britain, which has infuriated the Kremlin by giving asylum to tycoon and fierce Kremlin critic Boris Berezovsky and Chechen rebel envoy Akhmed Zakayev in addition to Litvinenko, a former Federal Security Service officer.

Chaika dismissed allegations that Russia could trade suspects in Litvinenko's murder for Berezovsky and Zakayev. He said that British authorities, which have rejected several Russian requests to extradite them, eventually would have to change their mind.

British experts also are conducting radiation checks in the British Embassy in Moscow, a spokesman said on condition of anonymity in line with embassy policy. He said the checks were a "a purely precautionary measure," similar to checks conducted in public places in London.

Meanwhile, a spokesman for former Russian Prime Minister Yegor Gaidar said Tuesday that Russian doctors had been unable to diagnose his mysterious illness. They say they suspect poisoning, but are unable to detect a toxic substance, aide Valery Natarov said.

Gaidar, a 50-year-old economist who served briefly as prime minister in the 1990s under President Boris Yeltsin and is a leader of a Russian liberal opposition party, began vomiting and fainted during a conference in Ireland on Nov. 24 — the day after Litvinenko died — and was rushed into intensive care at a hospital. He has been released from a Russian hospital.

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