Spanish courts sentenced 18 people for the bombings, which killed 191 people on two packed commuter trains on March 11, 2004, ruling that they were inspired by -- but not organised by -- Al-Qaeda.
But a new book by terrorism expert Fernando Reinares indicates the bombings were instigated by a senior Al-Qaeda member and the armed Islamist movement directly "approved and facilitated" them.
Reinares said that since 2008 he had been studying official documents from several countries and from Al-Qaeda itself and interviewing intelligence officials in Pakistan and elsewhere.
Reinares, a researcher at the Real Instituto Elcano international affairs institute and associate professor at Georgetown University in Washington, DC, was presenting his book to journalists ahead of its publication tomorrow.
"The decision to attack Spain was taken in December 2001 in Karachi," Pakistan, and was instigated by Amer Azizi, a senior Al-Qaeda leader and member of a cell set up in Spain by the network in 1994, Reinares said.