Mr Caballero, spokesman for the Navarran People's Union (UPN) group on Pamplona council, is the fifth local conservative politician to have been gunned down in the past year. His party is a regional ally of the ruling Popular Party. He had recently argued with councillors of the Basque nationalist Herri Batasuna party, supporters of the separatist organisation Eta. The HB councillors sued Mr Caballero for slander after he had accused them of "incitement to murder".
Yesterday's attack came moments after the Interior Minister, Jaime Major Oreja, announced the discovery of an Eta plot to kill King Juan Carlos. He said Eta planned to attack on the King during a visit later this year to the Basque city of San Sebastian. The plot, the third in recent years directed against the King, was uncovered during the interrogation of six Eta suspects arrested at the weekend.
Eta guerrillas had the King in their rifle sights in August 1995 during a failed assasination attempt on Majorca. Three suspects were convicted and jailed last year. Last October, police foiled a plot to bomb the inaugural ceremony of the Guggenheim museum in Bilbao which the King was due to attend.