ETA - which in the Basque language stands for Euskadi Ta Askatasuna, or Basque Homeland and Liberty - had been expected to announce its final dissolution this week following an earlier decision to disarm in April 2017, six years after declaring a permanent ceasefire.
The April 16 letter said that while ETA is now consigned to history, the drive for Basque independence would continue.
The Basque separatist group ETA is formally dissolving after 60 years and giving up its fight for an independent Basque state in northern Spain and southern France.
"We wish to show our respect for those who were killed or wounded by ETA and those who were affected by the conflict".
Speaking shortly before ETA's announcement Wednesday, the COVITE group that represents victims said it wanted more.
The group never met its political targets, and by 2009, a survey carried out by the University of the Basque Country found that only one percent of Basque respondents said they totally supported ETA. "ETA has completely dissolved all of its structures and declared an end to its political initiative".
But Spanish Interior Minister Juan Ignacio Zoido said on Wednesday that "they will not obtain a thing for making a declaration they call a dissolution". It was born in 1959 under the dictatorship of Spanish General Francisco Franco.
The group released a statement last month in which it apologized to the families of its victims, adding that it accepted "direct responsibility" for its crimes. To many people's surprise, the group continued its campaign of violence after the introduction of democracy and the 1978 constitution, which granted substantial autonomous powers to the Basque region.
The group has demanded that prisoners be moved to prisons closer to their families after the end of ETA, AFP news agency reported on Wednesday.