Spain is returning to Italy a suspect believed to have played a key role in last year's Madrid train bombings.
Rabei Osman Ahmed was arrested in Italy in June
Rabei Osman Ahmed, 33 - known as the Egyptian - was extradited to Spain for a maximum of six months in December. He also faces terrorism charges in Italy.
Judge Juan del Olmo, investigating the bombings, has filed provisional charges of mass murder and terrorism against Mr Ahmed, according to AP.
Bombs on packed commuter trains killed 191 people in Madrid on 11 March 2004.
The attacks are believed to have been carried out by a radical Islamist network based in Spain - mainly of North African origin. More than 70 people have been arrested in connection with the attacks.
Seven of the bombing suspects died when they blew themselves up during a police raid on the Madrid suburb of Leganes on 3 April 2004, including the alleged mastermind, a Tunisian named Serhane Ben Abdelmajid Fakhet.
Mr Ahmed was arrested in Italy in June after the authorities there intercepted telephone calls in which he allegedly boasted about his central role in the Madrid bombings, which he is alleged to have called "my project".
Italy agreed to extradite him to Spain for questioning for a maximum of six months. The Spanish Interior Ministry confirmed Mr Ahmed was due to be returned on Monday.
Spanish news agencies report that Mr Ahmed has left Spain on a flight to Milan.
Spain regards him as an explosives expert who had links with Serhane Ben Abdelmajid Fakhet.
According to the arrest warrant, Judge Juan del Olmo said Mr Ahmed "managed to take control of a small group of Arab followers, all of them with extremist Islamic ideology, supporters of jihad and Osama Bin Laden".
Mr Ahmed is also mentioned in charges of helping finance terrorist groups brought against 11 Pakistani citizens who were arrested in Barcelona last year.
One of the men is suspected of having sent Mr Ahmed up to 6,000 euros (£4,102).