The hunt for the remaining members of a terror cell wanted for the Madrid train bombings is being stepped up in the wake of the explosion at an apartment in a suburb of the Spanish capital over the weekend.

Spanish security forces are continuing to pursue leads in an attempt to capture several suspects who may have escaped the blast which killed the group's alleged ringleader, Serhane ben Abdelmajid Fakhet, and four associates.

Investigators searching the apartment discovered at least two rucksacks with explosives ready to be set off, indicating that the March 11 bombings that killed 191 people and injured 1800 was the start of a campaign rather than the end. The Interior Minister Angel Acebes said the haul of prepared explosives proved the cell had been poised to strike again.

Three suspects from a list of suspects are still at large: Moroccan brothers Mohammed and Rachid Oulad Akcha and Moroccan Said Berraj.

The blast, set off by the suspects as police closed in on an apartment in Leganes, south of Madrid on Saturday, also killed one policeman and injured 11 others, some seriously.

Suspects detonated bomb as police advanced

Spanish police were about to storm the building after investigations into the commuter train bombings three weeks ago led them to the three suspects. The police operation began when security officers approached the apartment but the situation soon escalated when the suspects spotted them from a window and they "started shooting from the apartment, chanting in Arabic," according Acebes.

Police then evacuated residents in the building and the surrounding area, and cordoned it off. A team then re-entered the apartment block and it was at this point, scoring to the Interior Minister, that the suspects set off a huge explosion in the apartment that the police were seeking to search, in what is believed to have been an attempt to resist arrest.

"The special police agents prepared to storm the building and when they started to execute the plan, the terrorists set off a powerful explosion, blowing themselves up," Acebes said. "There are three that blew themselves up, but the possibility of more is not ruled out.

Suspects believed to be of Moroccan origin

"Police believe some of the terrorists... could have been some sought by the security forces as the alleged perpetrators of the killings of March 11." The Interior Minister refused to identify the men, but Spanish media describe them as young Islamist militants of Moroccan origin.

Neighbours said a group of North Africans had moved into the apartment about a month ago. They were rarely seen and the blinds were always drawn, the neighbours said.

Last week, Judge Juan del Olmo issued an international arrest warrant for Fakhet, a man he described as the "leader and co-ordinator of the different people implicated" in the Madrid bombings.

The warrants named Fakhet as the man allegedly responsible for declaring a "jihad" or holy war in Madrid sometime in 2003. Fakhet "not only was the energising force for the awareness campaign for the jihad… but also with specific intent (since the middle of 2003 at least) for the preparation of a violent act in Spain, specifically in the Madrid area," Judge Juan del Olmo said.

Other warrants were also issued for five Moroccans who may have disappeared from Spain after the bombings. One of those wanted has been traced to an al Qaeda meeting in Istanbul in 2000; the others include the two brothers of the only woman jailed in the case, and a man who rented the house used to prepare the bombs.

North African link becomes stronger

Fakhet, also known as "The Tunisian" was the first person from that North African country suspected in the case. Spanish police have so far focused on suspects from Morocco after Spain's Interior Ministry identified the al Qaeda-linked Moroccan Islamic Combatant Group as the prime suspect in the Madrid attacks.

The group is believed to be the forerunner of Salafia Jihadia, which Morocco blamed for bombings in Casablanca that killed 33 people and 12 suicide bombers last year.

Spanish police are now holding 24 people as part of their investigations. Five have already been accused of crimes associated with the Madrid attacks after being brought before a Madrid judge a week after the attacks.