Three Moroccans and two Indians face a closed-door preliminary hearing on Thursday at what the government says is a "decisive phase" in the inquiry into the attack, which killed 201 and injured more than 1750 a week ago.
Meanwhile, Spanish police have arrested four more people and conducted a series of raids in connection with the blasts, sources at Spain's Supreme Court said on Thursday.
Three were arrested in Alcala de Henares, the suburb of Madrid where the bombs were thought to have been placed on packed commuter trains, and a fourth was detained in the northern town of Gijon.
Other court officials said the suspects were from North Africa but this has not been confirmed.
The latest arrests bring to nine the number of suspects detained over the attack.
The blasts helped provoke a major election upset, ushering in a Socialist leader who has vowed to stand by a pledge to pull Spain's troops from Iraq despite US calls to stay the course.
Incoming prime minister Jose Luis Rodriguez Zapatero branded a "fiasco" the occupation of Iraq which was backed by outgoing Prime Minister Jose Maria Aznar despite huge public opposition.
His critics say Spain's withdrawal would hand victory to the attackers and a new letter signed by a group that has claimed responsibility for the bombs will give critics fresh ammunition.
The Madrid bombings killed in
excess of 200 people
"Praise be to God who gave us this victory in the conquest of Madrid ... where one of the pillars of the axis of Crusader evil was destroyed," said a statement signed by the Abu Hafs al-Masri Brigades, which claims links to al-Qaida.
The letter, sent to a pan-Arab newspaper on Wednesday, said the group was calling a Europe-wide truce until it knew Zapatero's intentions over Iraq, where a suspected al-Qaida car bomb killed 27 people in Baghdad on Wednesday.
Zapatero, who opposed the conflict, was trailing in the polls before the Madrid blasts on four commuter trains, which Aznar instantly blamed on the Basque separatist group ETA.
Outrage over Aznar's handling of the attack and a revival of public opposition to the Iraq war swept Zapatero to power. He takes office next month.
Police are hunting some 20 Moroccans, who may also be linked to last May's Dar al-Baida (Casablanca) attack in Morocco in which 45 people died.
The five arrested suspects - including a Moroccan named as Jamal Zugam - are expected to return to jail after their court appearance on Thursday. An Algerian man picked up in the northern Basque city of San Sebastian is also being held.
Possible links investigated
Britain's Metropolitan Police and intelligence agencies are probing possible links between the suspects and Islamists living in London, El Mundo newspaper said on Thursday, citing police sources.
London's Metropolitan Police had no comment.
Spain is still grieving for its dead
Separately, Imad Ad-Din Barakat Yarkas, also known as Abu Dahdah - described as al-Qaida's leader in Spain and charged with a role in the 11 September US attacks - was due to appear in a different court on Thursday, El Pais reported.
The newspaper, quoting state agency EFE, said Dahdah had condemned the Madrid attacks in a letter to EFE on Monday.
Along with the rest of Europe, Spain has stepped up security at airports and at strategic installations in the face of what Aznar's government called "an ongoing terrorist threat".
Zugam ran a store that sold mobile phones in Madrid's multi-racial Lavapies district. Police believe mobile phones were used to detonate the bombs.
The official death toll remains 201 but a forensic scientist told El Mundo that figure was questionable.
She said the number of people reported missing exceeded the number of dead as many bodies had been blown to pieces.