Spanish authorities believe the bomb masterminds are dead or in custody

The latest suspects are reported to be from Morocco, Saudi Arabia and Egypt.

Eighteen people have been provisionally charged over the attacks, which left 192 people dead and more than 1,900 injured.

Six people arrested over the last few days because of their connections to people implicated in the bombings have been released.

Spain's incoming prime minister Jose Luis Rodriguez Zapatero said at his investiture that combating terrorism would be a key priority of his new government.

Of the suspects who have been provisionally charged, 14 are Moroccans, two are Indians, one is Spanish and another Syrian.

The Spanish authorities believe the "nucleus" of the Islamist group behind the attacks on the commuter trains was either destroyed by an explosion during a police raid on 3 April, or had been arrested.