BARCELONA, Spain — Two Spanish aid workers kidnapped almost nine months ago by an al-Qaida affiliate arrived Tuesday in Barcelona after a multi-million-dollar ransom was reportedly paid for their freedom — a sign of the terrorist group's growing sophistication in bankrolling operations through kidnappings, experts said.Aid workers Roque Pascual and Albert Vilalta were abducted last November when their convoy of 4-by-4s was attacked by gunmen on a stretch of road in Mauritania. They were whisked away to Mali, whose northern half is now one of the many stretches of remote desert where al-Qaida of Islamic Maghreb, or AQIM, has stretched its tentacles.RELATED:7 nabbed for cab-driver murderTop US security official: Our enemy isn't terrorism, jihadLate on Monday afternoon, the pair stepped out of a helicopter that landed on the grounds of the presidential palace in Burkina Faso and were handed a cell phone. Reporters overheard them saying into the phone 'muchas gracias' — or many thanks.Spanish newspaper El Mundo reported that Spain had paid euro3.8 million in ransom to secure the aid workers' release, however the government refused to comment. The two men arrived early Tuesday in Barcelona where they were greeted at the airport by family members and government officials. Vilalta, who suffered multiple bullet wounds to his leg when he tried to flee his abductors on the day of the kidnapping, walked with the aid of a single crutch."Now we are free and I'm very happy and very moved," he said. "For the rest of my life I will try to make up to you what I put you through," said Pascual, raising a clinched fist in the air in a sign of victory.Both men thanked the Spanish goverment for its diplomatic efforts to free them.Originally based in Algeria, AQIM had limited reach until 2006, when the organization, then called the Salafist Group for Call and Combat, brokered a deal with al-Qaida's leadership in the Middle East, allowing them to become in essence a franchise of the larger terrorist network.Since then they have abducted Austrian, Swiss, Italian, French and Canadian nationals. Experts say the majority were released after multimillion dollar ransoms were paid, money that is being reinvested to grow the group's footprint."It's clear that ransoms are being paid, since no political demand is usually made in connection with these kidnappings," said Pham, who is the senior vice president at the National Committee on American Foreign Policy and who recently traveled to Mauritania and Morocco in order to research the growth of the group."It would be illogical to assume that AQIM is carrying out these kidnappings and making no demands for their hostages. The dangerous innovation that we have seen in recent years is that the ransoms have gone beyond acting as startup money. It's now been incorporated into their business model and has become a major component of their strategy," he said.He pointed to the prisoner exchange that is believed to have taken place last week before the release of Pascual and Vilalta. Soon after they were kidnapped on Nov. 29, Mauritanian commandos led a raid across the border into Mali, where the terrorist group is believed to have an operating base. There they seized Omar Ould Sid Ahmed Ould Hama. He was sentenced by a Mauritanian court to 12 years in prison for taking part in the kidnapping of the aid workers.Associated Press writers Brahima Ouedraogo in Burkina Faso; Rukmini Callimachi in Dakar; Ahmed Mohamed in Nouakchott, Mauritania; Ciaran Giles in Madrid and Martin Vogl in Bamako, Mali, contributed to this report.