German police said on Monday that Dmitry Kovtun's family appeared to be contaminated with toxic polonium due to his visit to them.
Thomas Menzel, who leads a major police inquiry in Hamburg, told reporters that Kovtun's Russian-born ex-wife, their two children aged 3 and 1, and her new partner all showed signs of contamination.
Doctors were still trying to establish if the poison had entered their bodies, he said, adding the tests at a Hamburg hospital could take a few days.
German police said Saturday they had found "indications" of radiation in Hamburg in Kovtun's ex-wife's department. Kovtun is a business man who met former Russian spy Alexander Litvinenko in London on Nov. 1, the day the former Russian spy was believed to have fallen ill.
Previously police said the amounts of polonium-210 spread by Kovtun were so tiny that no one else could be contaminated.
Kovtun stayed for the night on a couch at his ex-wife's home before flying to London to meet Litvinenko, who was killed by the highly radioactive substance on Nov. 23.
Police said that they do not yet know if Kovtun was a poisoner or a victim of poisoning.
A team of 170 German police, helped by government radiation scientists, have been searching for smudges of polonium-210 left by Kovtun on car seats, beds, bathroom fittings and government documents during his stay in Hamburg from Oct. 28 to Nov. 1.
Germany is investigating whether Kovtun knowingly carried the toxic substance.
The German government said it had no separate information aside from the police inquiry.
A liaison officer from London police force, Scotland Yard, arrived Monday in Hamburg to assist the investigation.