From AP, with thanks to DC Watson:
The suspects included the security guard of a coal mine from which the 440 pounds of explosives used in the March 11 train bombings were stolen, Interior Minister Jose Antonio Alonso said.
Two other suspects were the wife and brother-in-law of Jose Emilio Suarez, a former miner in Asturias who already has been jailed and charged with multiple murders in the attacks, which killed 191 people.
The six suspects have not been officially charged with a crime but were suspected of supplying the dynamite, Alonso said. The brother-in-law had been arrested once before on March 27 but was released after questioning.
Alonso declined to give details of how the dynamite was passed on to the suspected Islamic cell that staged the attacks. But officials previously have said the bombers paid for the dynamite with cash and drugs.