At least three suspects in the Madrid railway bombings (search) blew themselves up Saturday as police prepared to storm their apartment. One special forces agent was killed in the explosion and 15 police officers were wounded.
The blast in Leganes (search), a southern suburb of Madrid, blew out part of the exterior walls on the first and second floors of the brick apartment building.
Police had approached the building at around 7 p.m. to make arrests as part of an escalating manhunt for those responsible for the March 11 bombings that killed 191 people and wounded more than 1,800.
The suspects spotted the police from a window and shot at them, chanting loudly in Arabic, the Interior Ministry said. No police officers were hurt by the gunfire.
Over the next two hours, police evacuated as many people as they could from the building and surrounding area and prepared for an assault on the apartment.
"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," Interior Minister Angel Acebes (search) said.
"There are three that could have blown themselves up, but the possibility of more is not ruled out," he said.
The news agency Europa Press said forensic experts were searching the building's swimming pool for remains of a possible fourth suspect, but the report could immediately be confirmed.
Police believe some of the suspects may have carried out the March 11 train bombings, Acebes said.
After the blast, floodlights lit up the wreckage in the exposed rooms of the building. Pieces of concrete littered the floors and wires dangled from the ceilings.
Leganes is a city of 175,000 people about 10 miles southwest of central Madrid.
The investigation into the March 11 attacks have focused on the Moroccan Islamic Combatant Group, which has links to Al Qaeda (search).
Judge Juan del Olmo, the investigating magistrate, has issued international arrest warrants for five Moroccans and a Tunisian, identified as Sarhane Ben Abdelmajid Fakhet and described as the leader of the bombers.
Another 15 suspects are already in custody. Six have been charged with mass murder and nine with collaborating with or belonging to a terrorist organization. Eleven of the 15 charged are Moroccan.
Earlier Saturday, Acebes said a bomb found under the tracks of a high-speed train line on Friday was made of the same brand of explosive, Goma 2 Eco (search), that was used in the Madrid train attacks.
However, he said it was still too early to name any suspects. Goma 2, often used for demolition and in mining, is relatively easy to get in Spain.
"It's the same type of explosive and it's the same brand," Acebes said of the 26-pound bomb. The bomb was planted about 40 miles south of Madrid, and its discovery stopped six bullet trains using the Madrid-Seville line.
Train service resumed Saturday, but soldiers, police and Civil Guard officers could be seen patrolling the targeted rail lines. Sunday is the start of Holy Week, when many Spaniards take vacation or travel to their hometowns for the Easter holiday.
Because the bag containing the bomb was dry and the ground was wet, authorities believe it was placed at the scene Friday. A 450-foot-long cable was attached to the detonator.
The rail line where the bomb was found mainly serves Spain's AVE bullet trains, which have a top speed of 190 mph, although some slower trains also use it.
Another bomb was found half-buried beneath a French railroad track on March 24 about 100 miles southeast of Paris, triggering a massive inspection of France's rail network. Authorities have not said whether they suspect any connection to the bombs in Spain.
The Spanish government's main suspect in the March 11 attacks, the Moroccan Islamic Combatant Group, is related to a group suspected in last year's Casablanca bombings, which killed 45 people including 12 suicide bombers.
Spain has been a major U.S. ally in Iraq and has been warned previously by Al Qaeda that it would be the target of terrorism for its support.
The Spanish newspaper El Mundo reported Saturday that the Spanish Embassy in Egypt received a letter from an Islamic militant group threatening new attacks if Spain did not withdraw its troops from Iraq and Afghanistan.
In the letter, the Brigade of Abu Hafs al-Masri, a group that also claimed responsibility for the March 11 attacks, threatened to strike against Spanish diplomatic missions in North Africa and the Mediterranean region unless Spanish troops are withdrawn in four weeks.
A Spanish diplomat in Cairo, speaking on condition of anonymity, confirmed that the embassy received a threatening letter signed by Abu Hafs after the commuter train attacks last month.
The United States believes the Abu Hafs group lacks credibility and has only tenuous ties to Al Qaeda. In the past, the group has claimed responsibility for events to which they were not connected — such as last summer's blackouts in North America and Britain.