Spanish police arrested a Russian citizen on Friday for allegedly playing a part in hacking the US presidential election last year.
The Russian embassy in Madrid on Sunday confirmed to Reuters the arrest of computer programmer Pyotr Levashov, who has been remanded in custody, although it did not say on what grounds.
Russian television station RT said that Levashov was subject to an international arrest warrant issued by the US government, while a "legal source" told AFP that he was subject to a US extradition order.
Suspicions have floated for months that Russia attempted to sway the 2016 presidential race. In January, US intelligence and security agencies said in a declassified report that Russian President Vladimir Putin ordered hacking attacks and propaganda campaigns in the run-up to the November election. The breaches included hacking of emails from the Democratic National Committee, Democratic candidate Hillary Clinton and her campaign manager, John Podesta.
Meanwhile, both houses of the US Congress are conducting their own investigations into potential influence peddling by Russia.
Security blogger Brian Krebs identified Levashov as an alleged spam king and distributor of malware.
The US Department of Justice and the Russian Embassy in Madrid did not immediately respond to requests for comment.