Bellingcat in its investigation released late Thursday 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 didn't immediately confirm the report, saying Friday that they were assessing an Associated Press request for comment.
Britain has charged Petrov and Boshirov with trying 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 to the arrests with attempts by them to smuggle hacking equipment to Switzerland.
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