Image handout dated March 14, supplied by the Spanish National Pólice showing part of the large cache of refurbished assault weapons, ready for their sale in the black market impounded during a joint operation EPA

Spanish police announced on Tuesday they had impounded over 10,000 combat weapons believed to be ready for sale on the black market to terror groups or mafias.

The weapons were impounded last January in the northern Spanish provinces of Vizcaya, Gerona and Cantabria during a joint operation with Europol, said a Spanish police spokesperson on Tuesday.

The operation was initially linked to an investigation into the weapons used by the gunman who on May 24, 2014 attacked the Jewish Museum in Brussels, killing four people.

Some time later, Spanish police arrested five people and seized hundreds of assault weapons.

The criminal organization had set up a sophisticated clandestine weapons factory that recovered and re-activated weapons previously purchased in the legal weapon surplus market as deactivated weapons, which they repaired and made operational after replacing the pieces that rendered it useless.

In all, the police have impounded over 9,000 CETME assault weapons of different models; nearly 1,000 assault weapons from manufacturers such as Mauser, Bergmann or Beretta; fully working mortar shells minus the explosive charge; anti-aircraft machine guns and pistols from the Star, Llama, Astra and FN companies and numerous revolvers.

Police also seized gun-making machinery and tools along with many spare parts such as barrels, bolts, magazines, pins and stocks.

These impounded weapons, their war-grade calibre and fully automatic gunfire, combined with the easy process of re-activation, made them an ideal choice for black market sales.