The British judge investigating the radioactive poisoning death of former Russian agent Alexander Litvinenko accused Russian authorities of interfering with his inquiry July 28.
Robert Owen spoke after suspect Dmitry Kovtun failed to give evidence by video link, saying he was bound by confidentiality obligations to an ongoing Russian inquiry.
Owen said that either Kovtun's offer of participation had been "a charade" or "obstacles have been put in the way of his doing so."
Kovtun and a second Russian, Andrei Lugovoi, are wanted by British police for allegedly poisoning KGB officer-turned Kremlin critic Litvinenko at London's Millennium Hotel on November 1, 2006 using tea laced with the radioactive isotope polonium-210.
Litvinenko died three weeks later and on his deathbed blamed Russian President Vladimir Putin for ordering his assassination — a claim Moscow denies.
Kovtun has denied murdering Litvinenko, contending he accidentally poisoned himself. Russia refuses to extradite either suspect.
British authorities say they have found evidence of Russian state involvement in the incident.
Owen plans to wrap up inquiry hearings this week and issue his findings by the end of the year.