A British judge has opened an inquiry into the death of former Russian agent Alexander Litvinenko, saying the central question is whether the Russian state ordered the killing, the Associated Press reports.

Litvinenko died in London in 2006 after drinking tea laced with radioactive polonium-210.

On his deathbed, Litvinenko accused Russian President Vladimir Putin of responsibility.

An inquest stalled over authorities' reluctance to disclose secret intelligence evidence. But this month — amid growing tensions with Moscow over violence in Ukraine — Britain announced a public inquiry into the death.

Opening the inquiry on Thursday, Judge Robert Owen said some hearings would be held in private to hear sensitive evidence. But he said "I intend to make public my final conclusions on the issue of Russian state responsibility."

Hearings are expected to start in January.