Four tonnes of bomb-making chemical was stolen from a French warehouse, police said on Saturday, and Spanish media said Basque separatists were suspected.
“It is a considerable theft,” a French police source said, refering to the amount taken.
Spanish state radio, quoting police sources, said investigators suspected the robbery of sodium chlorate fertilizer could be the work of the armed Basque separatist guerrilla group ETA, which is active in Spain and France.
Police have in the past confiscated caches of sodium chlorate they said ETA planned to use to make bombs.
The robbery took place on Friday in Saint Benoit in the western-central Poitiers region, according to the media reports.
Spanish state radio said six armed and hooded robbers tied up a security guard at the warehouse. They communicated using signs during the robbery but one of them spoke in French with a Spanish accent, reinforcing suspicions of ETA involvement, state radio reported.
It added no arrests had been made in the case.
ETA, classed a terrorist organisation by the European Union and the United States, has killed more than 800 people since 1968 in a bombing and shooting campaign for an independent homeland carved out of northern Spain and southwestern France.
Weakened by hundreds of arrests on both sides of the border in recent years, ETA has not killed anyone for two years.