Spain's National Court in 2007 sentenced Rafa Zouhier, 34, to 10 years behind bars for collaborating with the Islamist cell that carried out the country's deadliest-ever terrorist attack.
The court ruled that Zouhier had acted as the intermediary between a former Spanish miner who supplied the explosives and the leader of the cell that carried out the attacks, but did not know the use to which the dynamite would be put.
Although Zouhier, a former drug dealer turned police informer, was not convicted and sentenced for weapons trafficking until 2007, he had been behind bars since March 19, 2004.
Spanish police escorted him to Tangiers in northern Morocco immediately after his release in the early hours of Sunday from the Puerto de Santamaria prison in Cadiz in southwestern Spain, an interior ministry spokesman said.
Officers flanked Zouhier, who wore a black hooded sweatshirt and had his hands handcuffed behind his back, as they led him from a white police van into a small plane that took him to Morocco, a video released by the ministry showed.
Zouhier, a martial arts expert from Casablanca who moved to Spain when he was a teenager, was deported to Morocco under a provision in Spanish law that makes conviction for a serious crime grounds for expulsion.
During his trial, Zouhier declared himself to be "super innocent". He was expelled from the courtroom on four occasions -- including once for apparently failing to take the proceedings seriously when he nudged another defendant with his elbow.
Public prosecutors had asked for Zouhier to be jailed for 20 years for his part in the bombings.
Victims' groups welcomed his swift deportation.
"We are relieved, satisfied and happy because this risk to society is now in Morocco," Pilar Manjon, the president of the March 11th Victims' Association who lost her 20-year-old son in the bombings, told public radio RNE.
"He will no longer do shady deals with drugs in our country nor will he obtain explosives for any other attack," she added.
Zouhier married a 32-year-old Spanish computer sciences teacher in September 2013. According to daily newspaper El Mundo, they met at a nightclub where he worked as a bouncer before his arrest, and they had hoped to remain in Spain after his release.
Spanish courts have sentenced 18 people for the shrapnel-filled bomb attacks that killed 191 people and injured about 2,000 on four commuter trains heading for Madrid's Atocha station.
The coordinated attack was claimed by militants who said they had acted on Al-Qaeda's behalf in retaliation for Spain's involvement in the US-led invasion of Iraq.
The seven chief suspects committed suicide on April 3, 2004, by blowing themselves up in an apartment near Madrid, also killing a policeman.