The Spanish police arrested on Monday nine activists linked to pro-Catalan independence groups on suspicion they may have been preparing to commit violent acts, possibly with explosives, and Spanish prosecutors called for rebellion and terrorism charges to be leveled against them.

A statement from the Guardia Civil, the Spanish national police, said the operation in Barcelona Province was part of an investigation into the Committees for the Defense of the Republic, a grass-roots organization that supports Catalonia’s independence from Spain. In the past, the group has organized street protests and blocked road and rail lines.

The police said they carried out 10 raids and seized an abundance of material and substances they believe could be used to make explosives.

If confirmed, it would be a considerable blow to Catalonia’s mainstream independence movement, which is proud of its overwhelmingly peaceful approach since it began gathering momentum in 2011.