Two suspects in the poisoning of an ex-Russian spy were briefly detained in the Netherlands earlier this year, according to research by Bellingcat, an investigative group.
Bellingcat quoted an unnamed security official saying that Alexander Petrov and Ruslan Boshirov were detained in the Netherlands earlier this year and released without being charged.
Dutch authorities did not immediately confirm the report, saying on Friday that they were assessing a request for comment.
Britain has named Petrov and Boshirov as suspects in an attempt to kill Russian ex-spy Sergei Skripal and his daughter with the Soviet-designed nerve agent Novichok.
Media reports last week said two unidentified Russian spies were arrested on Dutch soil and deported, and leaked police information linked the arrests with attempts by them to smuggle hacking equipment to Switzerland.
The report also claims that Petrov had made previous visits to the UK in 2016 and 2017.