British investigators probing the poisoning death of former Russian agent Alexander Litvinenko on Monday visited a Moscow hospital that specializes in treating radiation cases, and Russian news agencies said they and their Russian counterparts questioned a key figure in the case, Andrei Lugovoi.
Another key figure in the case, Russian businessman Dmitry Kovtun, is believed to be staying in the city's Hospital No. 6. Lugovoi, a former KGB agent-turned businessman, may also be undergoing testing there.
The Interfax, ITAR-Tass and RIA-Novosti news agencies, citing an unnamed source, said British investigators and their Russian counterparts interrogated Lugovoi on Monday afternoon.
Lugovoi told ITAR-Tass that the questioning lasted three hours.
"I gave testimony exclusively as a witness. I was officially informed of that before the interrogation," ITAR-Tass quoted him as saying after questioning.
Neither the Russian Prosecutor General's Office, which is managing the British investigators' program in Moscow, nor the British Embassy would reveal any details of the investigation, the AP says.
An Associated Press photographer saw British Ambassador Anthony Brenton and some of the British investigators enter the Prosecutor General's office on Monday. The prosecutors' office later confirmed that Brenton had meet with Prosecutor-General Yuri Chaika, at the embassy's request.
"The ambassador expressed gratitude to the prosecutor general of the Russian Federation for the legal help given to the employees of New Scotland Yard in investigating the circumstances of the death of the former Russian (Federal Security Service) employee, Alexander Litvinenko," the office said in a statement on its Web site.
"Russian Prosecutor General Yuri Chaika, in his turn, noted the interest of the Russian Federation in the objective and multifaceted investigation of the facts of the death of Alexander Litvinenko."
A few hours after Brenton's meeting with Chaika, an Associated Press Television News cameraman saw British investigators enter Hospital No. 6.
It is the main medical center dealing with radiation in Moscow. The hospital treated victims of the Chernobyl nuclear disaster, and authorities have sent passengers who traveled on British Airways planes that traveled between Moscow and London - and showed traces of radiation - to Hospital No. 6.
The British investigators, who arrived in the Russian capital on Dec. 4, had tried all last week to interview Lugovoi without success, British law enforcement officials have said.
They and their Russian colleagues interviewed Kovtun last week. German investigators have been focusing on Kovtun after traces of polonium-210, the radioactive element that killed Litvinenko, were found in and around Hamburg.
Kovtun visited the city, where his ex-wife lives, on the eve of his Nov. 1 trip to London, where he and Lugovoi met with Litvinenko before attending a soccer match. Litvinenko said he fell ill that day and he died three weeks later. Russian prosecutors say that Kovtun has since been diagnosed with radiation sickness.
Lugovoi, Kovtun and a third associate who was in London with them on the weekend of Nov. 1, Vyacheslav Sokolenko, have denied involvement in Litvinenko's death.
On the second day of the St. Petersburg International Economic Forum, a plenary meeting was held, in which Russian President Vladimir Putin, French President Emmanuel Macron, Japanese Prime Minister Shinzo Abe, Chinese Vice President Wang Qishan and IMF head Christine Lagarde took part