After a weekend siege in suburban Madrid in which four or five suspected train bombers blew themselves up rather than give in to capture, a handful of suspected accomplices remained at large. 

"There could have been a series of Holy Week bombings, probably starting this weekend," a source close to the investigation said on Monday, adding that police were unsure how much of an arsenal remained in the fugitive bombers' hands. 

Al-Qaida letter

Investigators were analysing a letter purportedly sent by Usama bin Ladin's network to newspaper ABC that threatened more bombings unless Spain withdrew troops from Iraq and Afghanistan. 

"If our demands are not satisfied, we declare war and we swear by Almighty God that we will turn your country into an inferno and we will make blood flow like rivers," the letter said, according to an ABC Spanish translation. 

"In principle, the letter is given certain credibility, although the analysis is not yet complete," an Interior Ministry spokesman said on Monday. "We believe it could have been sent by people directly involved in recent events." 

The letter claimed responsibility for the 11 March attacks as well as a bomb planted on a high-speed rail line and defused on Friday. It said a previous truce with Spain ended as of midday Sunday. 

Arrests

The latest arrests took place on Saturday, one in the Madrid suburb of Fuenlabrada and the other in the Spanish territory of Ceuta on the north coast of Africa, a court official said. That took the number of suspects in custody to 17. 

Spain's interior minister said the
letter had 'certian credibility'

No information was available on their identities, nationalities nor what role they were thought to have played in the bombing. 

Interior Minister Angel Acebes said investigators would also step up their search for a possible mastermind or financier outside Spain with links to al-Qaida who might have ordered the 11 March attacks that killed 191 people. 

Madrid police patrolled the Madrid metro system on Monday for the first time, a job normally left to private guards. 

"Guaranteeing the safety of public transport is an absolute priority right now," Mayor Alberto Ruiz-Gallardon said. 
 
Silence

Spaniards marked five minutes of silence at midday to honour the special police agent who was killed at the weekend in the Madrid suburb of Leganes. A peace demonstration was called for Leganes on Monday night. 

"If our demands are not satisfied, we declare war and we swear by Almighty God that we will turn your country into an
inferno and we will make blood flow like rivers"

Al-Qaida letter

The Interior Ministry confirmed on Monday that train bomb suspect Jamal Ahmidan, one of six men for whom arrest warrants had been issued last week, was among those killed in Saturday's blast. Two others had already been identified, including the suspected ringleader, dubbed "El Tunecino" (The Tunisian). 

Besides the human bomb in Saturday's siege, at least two other bombs were packed in sports' bags in the apartment, and the body of one man recovered from a swimming pool was wearing an explosive belt of the type used by Palestinian bombers.