The two men suspected of poisoning an ex-Russian spy in Salisbury were briefly detained in the Netherlands earlier this year, an investigative group has claimed.
The report by Bellingcat quoted an unnamed security official saying Alexander Petrov and Ruslan Boshirov had been detained in the Netherlands, but were released without charge.
It also claims Mr Petrov had made previous visits to the UK in 2016 and 2017.
Britain has named Mr Petrov and Mr Boshirov as suspects in an attempt to kill ex-spy Sergei Skripal with the nerve agent Novichok.
The pair appeared in a video on Russia Today last week, claiming they had visited Salisbury as tourists to see the local cathedral.
They claimed a friend had recommended the city to them, and quoted a series of facts about the cathedral - including the height of its spire - that can be found on Wikipedia.
BBC News anchor Simon McCoy described the pair as "very keen on steeples".