Martin Sarasola and Igor Portu were arrested in Spain's Basque region on Sunday and found to be carrying hidden weapons.
Portu was admitted to intensive care shortly afterwards, suffering from a broken rib and a punctured lung.
The government said he was hurt resisting arrest. On Tuesday, Spanish police announced that they had found 125kg of bomb-making material in a cache discovered after the arrest of the two men.
The discovery, made in the northern province of Huesca, also contained detonators and timers, an interior ministry spokesman said on condition of anonymity.
The hidden materials were traced after police found a sketch map on one of the two detainees.