ETA has carried out attacks against companies working on the rail project
A businessman has been shot dead outside a restaurant in Azpeitia in the Basque region of northern Spain.
Spanish police say they suspect the Basque separatist group Eta is behind the killing.
Local media say Ignacio Uria Mendizabal, 71, worked for a company building a high-speed train link, a project previously targeted by Eta.
The shooting comes less than a month after the arrest of the suspected operational head of Eta.
Mikel Garikoitz Aspiazu Rubina, also known as "Txeroki", was arrested in southern France on 17 November.
The Spanish interior ministry had said it expected there to be retaliation for the arrest.
Officials say Mr Uria Mendizabal was shot several times in the car park of a restaurant in Azpeitia, near San Sebastian.
Media reports say he was shot in the head and the chest by several people who then fled the scene in a car.
Mr Uria Mendizabal was the head of the construction company, Altuna y Uria, that was working on a rail link in the region.
Eta has been blamed for three bomb attacks against companies working on the project already this year, though none caused casualties.
The group is opposed to the train link, which will connect the Basque cities of Vitoria, San Sebastian and Bilbao to the capital, Madrid.
The shooting happened a day before Spanish Prime Minister Jose Luis Rodriguez Zapatero was due to visit the region.