Spanish police are reported to have identified six Moroccans who they believe carried out the Madrid bomb attacks that have killed 201 people.
Spain is still deep in mourning for the victims
Five of the suspects are still at large but one is in custody, the Spanish newspaper El Pais quotes security sources as saying.
The man, named as Jamal Zougam, is reported to have been identified by people who survived Thursday's blasts.
Mr Zougam was arrested on Saturday with two other Moroccans and two Indians.
The number killed in the attacks has risen with the death of a 45-year woman. The figure is one short of the 202 people killed in Bali in October 2002 when a nightclub was bombed.
MOROCCANS IN SPAIN
Moroccans are the largest immigrant group in Spain
In 2003 there were 333,000, 20% of all legal immigrants
The number of illegal immigrants is unknown
Thousands cross the 13km (8 miles) Straits of Gibraltar every year on rafts or small boats
In 2003 24,146 people were repatriated to Morocco
Many work as cleaners, farm labourers or building workers
Polls show that Moroccans are Spain's least-liked immigrants
Security sources told El Pais that the six Moroccans might have formed only part of the group behind the attacks and that militants from other countries might also have been involved.
An interior ministry spokesman Juan de Dios Colmenero told the Associated Press that he could not confirm the reports in El Pais.
The BBC's Chris Morris in Madrid says the investigation is still in its infancy but there are already suspicions that the blasts could be linked to the leading Islamic militant, Abu Musab al-Zarqawi, who is wanted by the United States for a series of attacks in Iraq and elsewhere.
Meanwhile, police in the Basque city of San Sebastian have arrested an Algerian man who in January allegedly threatened to massacre people in Madrid, but initial reports suggest he is not a prime suspect.
The focus is falling increasingly on Morocco; Moroccan security officials are helping Spanish police.
The BBC's Pascale Harter in Tangier says there is great anger among Moroccans as the Spanish investigation seems to be leading back to their country.
A state-organised demonstration is due to take place in Tangier later on Tuesday, which is expected to be well attended.
People want a chance to express their solidarity with Spain, our correspondent says, especially after the funerals of a 13-year-old girl and a 24-year-old man from Tangier who were killed by the blasts.
As Spaniards also continue to bury their dead, a memorial service is to take place in Madrid's cathedral on Tuesday evening at 1900 GMT.
Officials have also announced that a state funeral for the victims will be held in Mardrid on 24 March.
The Spanish people are also continuing to digest Sunday's shock election result that saw the Popular Party turfed out of office. The Socialists, who won the biggest bloc of seats, are now trying to form a coalition with smaller parties to form a government.
Survivors of the attacks are reported to have identified Mr Zougam from photographs but police sources have said they are treating the witnesses' statements with caution.
One of the allegations against 30-year-old Mr Zougam is that he has links with the Salafia Jihadia group, held responsible for attacks in the Moroccan city of Casablanca last May that killed 45 people.
He is also said to have connections with Imad Yarkas, alias Abu Dahdah, the alleged leader of an al-Qaeda cell in Madrid, who is awaiting trial in Spain on charges of taking part in the 11 September plot.
Mr Zougam was detained with Mohamed Bekkali, 31, and Mohamed Chaoui, 34, all from Morocco.
Two Indians, named as Vinay Kohly and Suresh Kumar, were also arrested.
These five men were arrested in connection with a mobile phone which was found inside a bag containing explosives that failed to go off.
Investigators believe mobile phones were used to detonate 10 bombs hidden in backpacks on the four trains which were targeted.
Formal charges have not yet been presented.