A British judge opened an inquiry Tuesday into the death of Alexander Litvinenko, a former Russian intelligence agent who died from radioactive poisoning in London in 2006.

Litvinenko, who had become a Britain-based critic of the Kremlin, became violently ill in November 2006 after drinking tea with two Russian men at a London hotel. He died three weeks later.

Britain has accused Russia of involvement. Moscow denies the claim, and has refused to extradite the two men identified by Britain as the prime suspects.

"The issues to which his death gives rise are of the utmost gravity," Judge Robert Owen said. Some portions of the inquiry dealing with sensitive intelligence evidence will be held in private.

Owen has already said that he has seen secret British government material that "established a prima facie case that the Russian state was responsible" for Litvinenko's death.

The judge said the two Russian suspects — Dmitry Kovtun and ex-KGB agent Alexander Lugovoi — would be invited to give evidence by video link, "an invitation that I hope will be accepted." Both have denied responsibility for Litvinenko's death.

Litvinenko's widow, Marina, has said she hopes the inquiry, which is due to last several months, will reveal the long-buried truth about her husband's death. Owen said he hoped to publish his findings by the end of the year.

In an opening statement, the inquest's official lawyer, Robin Tam, said speculation about Litvinenko's death had swirled for eight years.

"Litvinenko himself blamed the Russian government," he said, while others had pointed to British agents, organized crime and the late Russian oligarch Boris Berezovsky.

"It has even been suggested that Mr. Litvinenko committed suicide," Tam said — but none of the theories has ever been tested in court.