LONDON — Traces of radiation have been found at a dozen sites in Britain during the investigation of the poisoning of a former Russian spy, Home Secretary John Reid said Thursday.

Reid also told Parliament that five jets are being investigated for possible radiation contamination. On Wednesday, British Airways said two of its Boeing 767s at London's Heathrow Airport tested positive and a third was grounded in Moscow awaiting examination.

The fourth aircraft, a Boeing 737 operated by the Russian airline Transaero, arrived at Heathrow Airport Thursday morning, Reid said, but he gave no further details.

He also said a fifth plane, also from a Russian carrier, was of interest to authorities, but he did not elaborate.

"There may be other airplanes of which we don't at this stage know," Reid said.

The search for contaminated sites has intensified since the Nov. 23 death in a London hospital of former KGB agent Alexander Litvinenko, a Kremlin critic who was found to have polonium-210 in his body.

Reid said "around 24 venues" have been or are being monitored and that experts had confirmed traces of contamination at "around 12 of these venues."

Reid said 1,700 calls had been made to the National Health Service, and 69 people were referred to the Health Protection Agency. Of those, 18 who may have been exposed to polonium-210 have been referred to specialist clinics, but all urine tests so far have been negative, he said.

The Health Protection Agency is expected to clear one of the three British Airways aircraft operating on the London-Moscow route that were identified Wednesday night as having possible radioactive contamination.

British Airways said "the risk to public health is low" but that it was attempting to contact some 33,000 passengers who have flown on the jets since Oct. 25.