LONDON - British detectives believe the radioactive substance used to kill former Russian spy Alexander Litvinenko cost more than US$10 million ($14.5 million).
According to the Times newspaper, preliminary results from the autopsy on Litvinenko show he was given more than 10 times the lethal dose of polonium-210.
Litvinenko fell ill on November 1, and died on November 23.
Several of his friends have blamed the Kremlin for the murder, but Russia has repeatedly denied any involvement.
"You can't buy this much off the internet or steal it from a laboratory without raising an alarm," an unidentified British security source told the Times.
"So the only two plausible explanations for the source are that it was obtained from a nuclear reactor or very well-connected black market smugglers."
The Times said British detectives in Moscow were due to return to Britain this week.
Citing unnamed sources, it said Russian officials had refused to ask questions of Andrei Lugovoi and Dmitri Kovtun - both of whom met Litvinenko on the day he fell ill - which British detectives wanted answered.
The detectives had not complained publicly, the newspaper added, because of the importance of the case to diplomatic relations between Britain and Russia.
