Madrid, April 5 (Reuters): Spanish authorities announced two more arrests in the Madrid train bombings today as a purported letter from al Qaida threatened more attacks that would “make blood flow like rivers” in Spain.

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 today, adding that police were unsure how much of an arsenal remained in the fugitive bombers’ hands.

Investigators were analysing a letter purportedly sent by Osama bin Laden’s Islamic militant network to newspaper ABC that threatened more bombings unless Spain withdraws 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 an ABC Spanish translation.

“In principle, the letter is given certain credibility, although the analysis is not yet complete,” an interior ministry spokesperson said today. “We believe it could have been sent by people directly involved in recent events.”

The letter claimed responsibility for the March 11 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 yesterday.

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. 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 may have ordered the March 11 attacks that killed 191 people.

Madrid police patrolled the Madrid metro system today 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. Spaniards marked five minutes of silence at midday to honour the special police agent who was killed in the weekend suicide bomb in the Madrid suburb of Leganes.

Meanwhile, Spanish troops continued to come under mortar fire today.