Lugovoi, a businessman and former security agent, was questioned by Russian prosecutors and Scotland Yard officers in hospital Monday.
"Investigators could have additional questions after analyzing Lugovoi's testimony, and then Scotland Yard officers and Russian prosecutors will interview him again," the source said.
Litvinenko, an outspoken critic of President Vladimir Putin's administration and a close associate of fugitive oligarch Boris Berezovsky, defected to the UK in 2000. He died in a London hospital November 23, a month after receiving British citizenship. A lethal dose of the radioactive polonium-210 was found in his body.
Russian prosecutors launched a criminal investigation on December 7 in parallel with the Scotland Yard-led probe into Litvinenko's killing, following a related murder attempt allegedly made on one of his Russian contacts, Dmitry Kovtun, another KGB agent-turned-businessman.
Following Monday's questioning, Lugovoi refused to reveal what he had been asked, but said that he was questioned as a witness, not as a suspect.
However, an insider source said investigators were interested in when, where, and with what purpose Lugovoi met with Litvinenko on November 1, which Litvinenko believed to be the day of his fatal poisoning.
