Five men arrested in connection with Madrid's suspected al-Qaeda bombings will appear in court later today.
Three Moroccans and two Indians face a closed-door preliminary hearing at what the government says is a "decisive phase" in the inquiry into last week's attack, which killed 201 and injured more than 1,750.
Police are still hunting 20 Moroccans in connection with the attack and police believe they may also be linked to last May's Casablanca attack in which 45 people died.
The five arrested suspects - including a Moroccan named as Mr Jamal Zougam - are expected to return to jail after their court appearance. An Algerian man arrested in the northern Basque city of San Sebastian is also being held.
The Spanish government's reaction to the blasts is thought responsible for it losing power in last Sunday's elections, which saw socialist Mr Jose Luis Rodriguez Zapatero confound the polls and prevent the People's Party from gaining a third successive term in office.
The Spanish government attempted to blame the attacks on the Basque separatist group Eta, but it is now widely held that an al-Qaeda-linked organisation was responsible.
Mr Zapatero, who has vowed to stand by his election pledge to pull the country's troops out of Iraq, has described the US-led operation as a "fiasco".
Police and intelligence agencies in Britain are investigating possible links between the suspects and extremist Islamists living in London, the El Mundonewspaper said today.