(CNN) ETA, the Basque separatist group that killed around 850 people during its half-century campaign for independence from Spain, announced its full dissolution, Spanish media reported.

The declaration was made in a letter dated April 16 and sent to a number of political institutions, Spanish news agency EFE reported. The announcement had been expected, and came just over a year after the group handed over its weapons, following a 2011 ceasefire.

ETA, which stands for "Euskadi Ta Askatasuna" or "Basque homeland and freedom," was founded in 1959 in response to the frustration felt by Basques during the repressive regime of General Francisco Franco. Its aim was to gain independence from Spain and establish a state in the north of Spain and southwest France.

"We know that we caused a lot of pain during that long period of armed struggle, including damage that can never be put right," it said.

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