British authorities have given the all clear to one of four aircraft being monitored for radiation as part of the investigation into the death from poisoning of a former Russian spy.
British Airways had said "very low traces" of a radioactive substance had been found on two planes being held at London's Heathrow airport but that one of the planes had been cleared by health officials last night to fly again.
The Health Protection Agency said passengers had not been at risk of radiation poisoning.
Officials are now checking the second plane at Heathrow, while a third BA craft in Moscow will be flown back to London today to undergo tests. All three aircraft had travelled between the Russian capital and London.
A fourth plane, a Boeing 737 leased by Russian carrier Transaero that landed at Heathrow yesterday, was also given the all clear after being checked by police.
"There is one other Russian plane, that we know, that we think we would be interested in," Reid said. "There may be other airplanes of which we don't at this stage know."
The announcement about the planes and their destinations could rekindle suspicions of a Moscow link to the poisoning.
Alex Goldfarb, a close friend of Litvinenko, told reporters in London he suspected the radiation had come from Russia.
"Alexander was convinced Mr Putin was personally involved in this," Goldfarb said as an inquest into Litvinenko's death was opened in London and adjourned.
In Moscow, Anatoly Safonov, Putin's counter-terrorism adviser, told Reuters: "As we said before, we are open and willing to offer all the help needed."
In a separate development, Russian doctors said they had identified no natural cause for an ailment afflicting former Russian Prime Minister Yegor Gaidar, who fell ill during a trip to Ireland the day after Litvinenko died.
Doctors said it was too early to say whether he had been poisoned, according to his spokesman Valery Natarov.
Gaidar, an academic who has voiced restrained criticism of Putin's economic policies, is recovering in a Moscow hospital.
In Britain, some 7,500 BA passengers have called a helpline seeking health reassurances after the airline's announcement.
BA says the health risk is low. It faces a huge task tracing 33,000 people who used the planes over a five-week period.