A British judge on Thursday opened an inquiry into the death of former Russian agent Alexander Litvinenko, saying the central question is whether the Russian state ordered the killing.
Judge Robert Owen said he was determined to publish conclusions about Russian responsibility, although some parts of the inquiry, and possibly some of his report, will remain secret.
Litvinenko, a former Russian FSB agent who became a critic of the Kremlin, died in 2006 after drinking tea laced with radioactive polonium-210 at a London hotel.
On his deathbed, Litvinenko accused President Vladimir Putin of responsibility.
British police have named two Russian men as suspects, but Moscow refuses to extradite them.
An inquest into the poisoning stalled because British authorities were unwilling to disclose secret intelligence evidence. But amid growing tensions with Moscow over violence in Ukraine, Britain this month announced a public inquiry.
The inquiry has the power to summon witnesses and documents from authorities in Britain, though not from Russia.
Owen said the issue of Russian state responsibility was of "central importance" to the inquiry. It is not due to examine whether Britain failed to protect Litvinenko, whose family believes he was working for
Britain's MI6 intelligence service before he died.
Owen said he would consider broadening his investigation if evidence emerged that Britain could have prevented Litvinenko's death.
Owen said some hearings would be held in private to hear sensitive evidence.
"But I make it clear now that I intend to make public my final conclusions on the issue of Russian state responsibility together with as much as possible of my reasoning in that regard," he said.
Inquiry hearings are expected to start in January, more than eight years after Litvinenko died.
His widow, Marina Litvinenko, said she was confident Owen would find out who was to blame.
"Everybody all around the world will know the truth," she said.