The Madrid attacks were the worst of their kind in Europe
The Spanish interior ministry says it is investigating reports that two suspects in the 11 March Madrid train bombings were police informants.
The move came after Spain's El Mundo newspaper said Moroccan Rafa Zuher and Spaniard Jose Emilio Suarez had been in contact with police before the attacks.
The men are suspected of providing dynamite for the attacks, which killed 191 people and injured more than 2,000.
The paper said they passed on details about drug deals and other crimes.
Mr Suarez, a former miner, was arrested a week after the attacks and is the only Spanish-born suspect in custody.
According to El Mundo, he was an informant for the National Police, providing information about trafficking in weapons, drugs and explosives.
The paper said Mr Zuher, who was arrested later in March, had passed on information to the Civil Guards in Madrid about low-level drug deals involving hashish and ecstasy.
El Mundo, citing security sources for its report, said Mr Zuher was believed to be the link between Mr Suarez, who allegedly supplied the explosives, and the cell that carried out the attacks.
After the report was published, the Spanish interior ministry issued a statement saying it had ordered an investigation.
The ministry said that if necessary, the results of the inquiry would be handed over to a judge.