______ was believed to be responsible for the blasts.

Four small bombs exploded Sunday at industrial sites in the Basque region in a string of attacks that appeared to have been staged by the armed separatist group ETA, Basque police said.
  Two policemen and a security guard suffered minor injuries after inhaling toxic fumes at a chemical plant where one of the pre-dawn blasts occurred, police said.
  The explosions came a day after Spain’ s ruling Socialist party said it is seeking other parties’ support for a parliamentary motion supporting the start of talks with ETA if the group renounces violence, although the negotiations would rule out concessions toward ETA’s goal of Basque independence.