A cloud of radioactivity which spread across Europe was believed to have been released from a steel mill in the south of Spain.
The detection of caesium 137 was noticed in France and Switzerland before being discovered by the Spanish authorities.
The cause appears to be the melting down of medical equipment with the caesium which was mixed in with other scrap metal.
It is believed that the scrap was screened for activity, but in this case, the active material was still in its shielding. The steel mill had detected increased radiation levels in the steel mill chimney.
The Spanish nuclear safety regulatory authority said that although the level of the radioactivity was too low to cause concern, it has launched an investigation into the causes of the incident.