MOSCOW, December 11 (RIA Novosti) - A key witness in the case of murdered former FSB agent Alexander Litvinenko has refused to reveal what Russian prosecutors and Scotland Yard officers asked him.

"It is a secret of the investigation and I cannot divulge such information," businessman Andrei Lugovoi told RIA Novosti, adding that he gave comprehensive answers to questions he was asked and that he is ready to meet with law enforcement officers once again if need be.

However, an inside source said investigators were interested in when, where, and with what purpose Lugovoi met with Litvinenko.

Litvinenko, an outspoken critic of President Vladimir Putin's administration and a close associate of fugitive oligarch Boris Berezovsky, defected to the U.K. in 2000. He died in a London hospital November 23.

A lethal dose of the radioactive substance polonium-210 was found in his body, as well as in places he visited.

Russian prosecutors launched a criminal investigation December 7 in parallel with the Scotland Yard-led probe into the killing of Litvinenko, following a related murder attempt allegedly made on one of his Russian contacts, Kovtun.

Lugovoi and his business partner Dmitry Kovtun were hospitalized and screened for radiation. Lugovoi had gone to see the former Russian agent in London together with Kovtun.

Some media have been reporting since Thursday that Kovtun's health has drastically deteriorated, but Lugovoi said Kovtun's condition is stable.