New forensic evidence on the bombings has raised an uncomfortable question for Spaniards. Is Osama bin Laden dreaming of exacting revenge for the loss of Al-Andalus, the ancient Moorish kingdom in Iberia?
A group close to bin Laden's al-Qa'eda network, the Brigade of Abu Hafs al-Masri, sent a message to a London-based Arabic newspaper explaining the reasons for attacking Spain.
"This is part of settling old accounts with Spain, the crusader and America's ally in its war against Islam," the statement said.
While the authentiticy of the message is open to doubt, there is no question that it reflects the thinking of Islamists, who hold that any land which has once been part of the Muslim community should forever remain under Muslim rule.
At the beginning of the 11th century, three quarters of Spain's population was Muslim but, as soon as the Catholic monarchs Ferdinand and Isabella completed the reconquest of the country for Christianity, the Muslims were ordered out.
The humiliation has never been forgotten in the Arab world.
The sense of hurt has grown since Spain, for decades a friend of the Arab world, backed the US-led war on Iraq, despite vast domestic opposition.
A dozen al-Qa'eda-linked suspected terrorists have been arrested in Spain from among its burgeoning community from North Africa.
Bin Laden has identified Spain as a worthy target, and the "settling of old accounts" will send a tingle down many an old Spaniard's spine. Bin Laden gave warning that Spain would be singled out for attack in a taped message released last October through al-Jazeera, the Arab satellite channel.
He said Spain would be among six nations considered "special" targets for its role in the Iraq war. Bin Laden has also spoken of Al-Andalus, regarded with nostalgia by Islamists as the halcyon age of Muslim power and artistic achievement.
The tale of the "Moor's last sigh" is recounted to epitomise the loss of one of the Islamic world's great jewels.
When King Boabdil fled the city of Granada, the last bastion of Moorish rule, he looked back and wept. His mother chided him with words that have sent a painful message down through the ages to Muslims: "Do not weep like a woman for what you could not defend like a man."
Moorish armies from North Africa conquered the Iberian Peninsula in the 8th century and transformed the region into an integral part of the Muslim umma, or nation.
The year 1492, when Granada was ceded to Ferdinand and Isabella, is a talismanic date for some Islamist scholars who consider it as the beginning of the decline of the Muslim world which continues to this day.
Most Europeans see the years of Islamic rule in Spain as a period of scholarship, leisured lifestyle and inspired architecture, though Islamists would focus on the vibrancy of the faith which allowed the Muslim armies to force their way deep into Christendom.
The jewels of Islamic Cordoba and Granada are still claimed by some who resent the expulsion of the Muslims from the peninsula. None of this proves that bin Laden was behind the bomb attacks on Thursday. An emotional attachment to a place does not automatically mean readiness to blow up innocent commuters.
If there is an Islamic connection, it may owe more to the availability of active cells and explosives.
But there is no doubt that the mere suggestion of an Islamist connection has sent a shudder through Spaniards.
Last year after a wait of more than 500 years, Spanish Muslims succeeded in building a mosque in the shadow of the Alhambra, once the symbol of Islamic power in Europe.
Perhaps Thursday's attacks were another return to the past, in this case a revival of Spain's ancient animosities.