MADRID — ETA, the Basque separatist group, is dissolving itself, it stated in a letter published on Wednesday, closing a history that included one of the longest terrorism campaigns in modern Europe, which killed over 800 people in Spain.

Militants of ETA had been expected to make a formal announcement some time this week, after almost six decades of fruitless and violent resistance to the central government in Madrid.

Spanish news organizations reported on Wednesday that in a letter to various officials and organizations, dated April 16, the group wrote, “ETA has completely dissolved all its structures and has terminated its political initiative.”

But the letter, first obtained by a Basque journalist at the online publication Eldiario.es, insisted that the Basque conflict with Spain — and, to a lesser degree, with France — “did not begin with ETA and does not end with the end of ETA’s journey.”