A decade since the deadly al-Qaeda-inspired train bombings in Madrid, Spain is again on alert against growing numbers of Islamist "lone wolves" willing to launch fresh attacks, officials say.
The ranks of young radical Islamists in Spain have swelled, recruited not in mosques but in internet chat rooms and private houses, officials and experts say.
Spanish courts sentenced 18 people for the bombings that killed 191 people on commuter trains heading for Madrid's Atocha station on March 11, 2004.
The Spanish anti-terrorist service's level of alert has since remained at "a likely risk of attack", junior security minister Francisco Martinez said.
"That has not changed ... but the number of jihadists has grown," he said. "Especially in certain areas, radicalisation has increased."
A study by the Royal Elcano Institute, a Spanish research body, said 84 Islamists, all young men, were convicted over attack plots in Spain between 1996 and 2012, or died in relation to such attacks.
Those who died were the seven chief suspects of the Madrid bombings, who committed suicide weeks afterwards. The convicts also included those seized in a failed plot in Barcelona in 2008.
Most of those Islamists were first-generation immigrants from Algeria, Morocco or Pakistan.
Increasingly, such suspects were being radicalised on the fringes of the Islamic world, not in the closely watched mosques, said Fernando Reinares, a security specialist at the institute.
"They tend to gather in small, marginal places of worship and in private homes."
This recent breed of extremists was marshalled not so much by Islamic clerics as by seasoned warriors, "charismatic individuals who have fought in Afghanistan, Bosnia or Chechnya", he said.
The number of people arrested as suspected terrorists fell considerably after the year following the Madrid attacks.
"After March 11, Spain was in a state of shock and judges were ready to authorise actions at the slightest indication," said Javier Jordan, a security expert at Granada University.
Of the 500 suspected Islamic extremists arrested between 1995 and 2014, only 78 have been convicted.
"Many of the people arrested then were freed for lack of proof," Jordan said. "The courts and the police are much more cautious now."
In recent years, however, police have arrested several suspects that Spain's interior ministry has identified as a new breed of "lone wolf", who have been self-radicalised online.
Last June, Spanish police in Ceuta, a Spanish territory on the northern tip of Morocco, broke up a gang that recruited and indoctrinated young men and sent them to fight in Syria.
The Elcano Institute in a report cited police intercepts of conversations between members of that network willing to "wage jihad at home", in Spain, if they returned from Syria.
"The risk is that individuals with European passports who have taken part in jihadist activities in Syria return to their country of origin with the intention of carrying out what they call acts of jihad," Reinares said.