Those returning are believed to have been trained in terrorist camps in Syria and Iraq, Spanish anti-terrorism forces told their European colleagues following the attack on the Amsterdam-Paris train on Friday by Ayoub El-Khazzani.
The potential terrorists “are not especially well trained”, but “they are prepared to do anything and once they have been indoctrinated, they receive minimal training in the use of weapons”, the radio station Cadena Ser quoted security experts saying.
Meanwhile on Tuesday Francois Hollande warned France must brace for further attacks. “We are still exposed,” Mr Hollande told diplomats gathered in Paris.
“The aggression that took place on Friday… which could have degenerated into a monstrous carnage… is fresh proof that we must prepare for other attacks and therefore protect ourselves,” said Mr Hollande.
Khazzani was a 25-year-old Spanish resident who apparently travelled to Istanbul in May and is believed to have received training from Isil.
Moroccan-born Khazzani appears to have planned to kill several passengers after carrying plenty of ammunition on the Thalys high-speed train, although he has told French investigators that his intention was armed robbery.
Khazzani and his brother Imran were monitored by Spanish security forces until they both left Spain in 2014. He went to France where he had apparently been offered a job, while Imran El-Khazzani was deported to his native Morocco.
It was officially because of irregularities in his residency papers, although it seems that the authorities were concerned at his potential to radicalise Muslims in his role as an imam in an Algeciras mosque.
Francisco Martínez, Spain’s secretary of state for security, highlighted the speed at which some young people sign up to Isil’s cause, earlier this month.
“In a question of weeks they become fanatics who travel to warzones or stay in their home areas to act as lone wolves.”
Fourteen Isil suspects were arrested in Spain and Morocco after a joint operation between the two countries’ security forces, Spain’s interior ministry said on Tuesday.
The arrests were made in San Martin de la Vega, near Madrid, and several locations in Morocco, including Casablanca and Fez, and the suspects are alleged to belong to a recruitment network for Isil’s camps in Syria and Iraq.