Spanish officials have said bullets used in a gun attack in the north of the country in which two policemen were killed were not a type normally used by ETA Basque separatist guerrillas.
The two officers died when shots were fired as they tried to stop a vehicle in the Navarre region, which is claimed by ETA as part of a Basque homeland.
Spanish state radio said an organised crime group may have been behind the incident.
However, a senior government official said no group could be ruled out.
