In a coordinated operation involving authorities in at least four countries, police in Belgium and Italy detained 17 radicals suspected of having links to al Qaeda, including a man investigators called a prime architect of the train bombings in Madrid in March.
Fifteen suspects were taken into custody Monday night and Tuesday in raids involving as many as 200 Belgian police officers in Antwerp and in the Brussels suburb of Schaerbeek, police told reporters in Brussels.
The federal police director, Glenn Audenaert, said the detainees included Egyptians, Moroccans, Palestinians and Jordanians. He said the group was planning an attack in another location.
Police in Milan, Italy, raided a half-dozen locations and arrested two men, including Rabei Osman Sayed Ahmed, 33, known to Spanish authorities as "Mohamed the Egyptian."
Spanish investigators said Ahmed was a former army explosives expert who conducted training courses at al Qaeda camps in Afghanistan during the years of Taliban rule. They said he was in Spain in 2003 and was in contact with the alleged ringleader of the March 11 train bombers, Sarhane Ben Abdelmajid Fakhet, known as "the Tunisian."
Fakhet, 35, was among seven people who blew themselves up in an apartment in the Madrid suburb of Leganes on April 3 after police surrounded it.
According to Spanish authorities, Ahmed recruited Fakhet at a Madrid mosque and perhaps supplied the explosives expertise for the train attacks. "He was involved with the Tunisian," a Spanish Interior Ministry spokesman said. Ahmed is thought to have left Spain in the months before the attacks and was traced to Italy through intercepted telephone calls.
In one such call, according to Spanish radio station Cadena SER, Ahmed was overheard speaking of Fakhet and the six others who died in the Leganes apartment, saying: "Those in Spain are my friends, but I am sad because I could not go to heaven with them."
Spanish media reported that Ahmed was arrested after he was identified by someone who could place him at the house in Spain where the bombs used on the trains were prepared.
Italy's interior minister, Giuseppe Pisanu, said Ahmed was "probably among the principal authors" of the plot to bomb the Madrid trains. The bombs killed 191 people, the highest toll in a European terror attack.
Italian authorities said a Palestinian identified as Yahia Payumi was also detained in the house, which he shared with Ahmed.
Investigators told reporters in Italy that the two had been under strict surveillance for several weeks and that wiretaps indicated another terrorist plot was in the planning stages. In wiretapped conversations, Ahmed was heard saying several times over the phone that he was ready to die for Allah.
"This group was ready to spread more terror and shed more blood in other attacks," Pisanu said later on RAI state television.
The detentions point to a new level of cooperation among European police and intelligence agencies. Bureaucratic strictures and cultures of secrecy have long inhibited the sharing of sensitive information, but governments have worked since the March 11 attacks to cooperate more closely.
Authorities have not yet disclosed full details of the cases against the suspects. In the past, European police have often arrested groups of alleged radicals, only to see them acquitted or quickly released for lack of evidence.
Josep Ramoneda, a terrorism expert in Barcelona, said the raids showed the authorities have made progress since March 11. But he said that among Spain's North African population there was a loose underground Islamic organization of about 3,000 people, including terrorists, collaborators and sympathizers who could supply indirect assistance.
Spain began proceedings to have Ahmed extradited there, where court officials said he would face 190 counts of murder, 1,430 counts of attempted murder and four counts of terrorism in connection with the train bombings.
Belgian authorities would not say whether the 15 people arrested in the raids there were directly linked to the March 11 bombings. But Spanish officials said at least two of them, a Palestinian and a Jordanian, were known lieutenants of Ahmed and were believed to have been involved in the March attacks.
Special correspondents Maria Gabriella Bonetti in Paris and Sarah Delaney in Rome contributed to this report.