Police on Friday arrested eight Spaniards who they said had returned from fighting alongside pro-Russian forces in eastern Ukraine, sources close to the investigation said.
Officers detained the eight suspects in six regions across Spain, the interior ministry said in a statement. A source described them as "pro-Russian" and far-left in ideology.
"Eight Spanish combatants have been arrested who had returned from Ukraine after taking part in the conflict in the Donbass region, in which they are accused of various crimes", the government statement said.
The source close to the investigation said the charges could include possession of weapons of war, complicity in homicide and violating Spain's neutrality in the Ukraine conflict.
Pro-Russian forces in the eastern Donbass region are battling those of the Ukrainian government, which is backed by the West. The conflict has killed at least 5,800 people since April.
The conflict has raised tensions between the West and Moscow to heights reminiscent of the Cold War.
The source said the Spanish suspects were inspired by the International Brigades, the multinational volunteer forces that fought against Francisco Franco's uprising during the Spanish Civil War.
The source said the combatants had posted online pictures of weapons and messages about their commitment to fighting in Ukraine.
In one web post dated August 2014, a Spanish left-wing group, invoking the example of the International Brigades, called on supporters to go to eastern Ukraine to fight against "the cancer of fascism" that it said was spreading in Ukraine.
Police were continuing searches, the interior ministry said.