(Vasily Dyachkov ~ Associated Press file)
Andrei Lugovoi is facing murder charges in the death of Alexander Litvinenko, a former KGB agent turned Kremlin critic whose tea was allegedly poisoned with a radioactive substance at a London hotel where he met with Lugovoi.
Prime Minister Tony Blair's office urged Tuesday that international law be respected, but Russia said a law prohibiting the extradition of its nationals trumped any international agreement.
"Murder is murder; this is a very serious case," Blair's spokesman said, speaking on condition of anonymity in line with government policy. "The manner of the murder was also very serious because of the risks to public health."
Radioactive traces were found at a dozen sites across London after Litvinenko's death Nov. 23, including three hotels, a soccer stadium, two planes and an office building used by self-exiled Russian tycoon Boris Berezovsky. More than 1,000 people in Britain and abroad were tested for polonium-210 contamination.
On his deathbed, 43-year-old Litvinenko accused President Vladimir Putin of being behind his killing. He had also accused Russian authorities of being behind a 1999 apartment blast and the shooting death of investigative journalist Anna Politkovskaya.
The Russian government has denied involvement in Litvinenko's death.
A senior U.S. intelligence official, speaking on condition of anonymity because of the sensitivity of the case, said the U.S. intelligence community is divided over why Litvinenko was killed, and who may have ordered his slaying.
Some intelligence analysts, the official said, think Litvinenko -- as well as Politkovskaya, who was killed in Moscow in October -- were killed because of their roles as critics of the war in Chechnya.
Others think Litvinenko's killing was part of a campaign against the Kremlin's political foes in the run-up to Russia's parliamentary elections in December and the presidential contest in March 2008. Another theory is Litvinenko was the victim of a personal feud.
While Russia has changed in many ways since the Soviet era, the official said intelligence analysts still watch for subtle signs to read the intentions of Russian leaders. Russia, the official said, "is just as much of a black box now as it was then."
Relations between Britain and Russia have long been sour.
Britain has refused to hand over Russian exiles, including Berezovsky -- once an influential Kremlin insider who fell out with Putin and fled to Britain in 2000 to avoid a money-laundering investigation -- and Chechen opposition leader Akhmed Zakayev.
It has also complained of growing numbers of Russian spies in Britain. Russia last year passed a law that allowed for security forces to use force abroad against people considered threats.
Russia's Federal Security Service, meanwhile, accused four British diplomats of spying after a state-run television report said British diplomats had contacted Russian agents using communications equipment hidden in a fake rock in a Moscow park.
"This is the latest wrinkle in our relations," said a British government official who spoke on condition of anonymity because of the sensitivity of the case. He said he doubted Britain would back down.
Britain's ambassador to Russia, Anthony Brenton, has also complained he is being stalked by a pro-Kremlin Russian youth group called Nashi who wanted him to apologize for participating in an opposition conference last year.
The most delicate issue, however, is energy.
Britain exports oil and gas but depleting supplies have raised concerns about future reliance on the Gulf states and Russia.
The EU gets a third of its oil and about 40 percent of its natural gas from Russia.
One-fifth of the world's gas reserves are in Russia and are controlled by Gazprom, the giant Russian utility. Gazprom, which already has a minor presence in Britain, is targeting 20 percent of the domestic gas market by 2015.
"It's foolish," said London-based ex-U.S. intelligence officer Bob Ayers. "Russia is becoming a monopoly when it comes to energy supplies in Europe and the last thing you want to do is jeopardize that supply."
Lugovoi joined the KGB in 1987 after serving in the Kremlin guard corps. During his time in the KGB, he provided security for Prime Minister Yegor Gaidar, among others.
He also headed a group of guards for Berezovsky, who at that time was deputy head of the Russian Security Council, and later headed the security corps for ORT television, Russia's most widely broadcast channel. He now has a security company and interests in the production of the Russian drink kvas.
"I consider this decision to be political, I did not kill Litvinenko," Lugovoi said. "I have no relation to his death and I can only express a well-founded distrust for the so-called basis of proof collected by British judicial officials."
Lugovoi could be tried in Russia, prosecutors said. Litvinenko's widow, Marina, dismissed such a scenario.
"Everything that happened, happened here," she said.
Associated Press Writer Douglas Birch contributed to this report from Moscow.