The group had been expected to announce its final dissolution later this week, after its ultimately unsuccessful drive for an independent state in northern Spain and southern France that killed 853 people.
THIS advocated for the independence of ethnic Basques from Spain and France.
Eta, deemed a terrorist organisation by the European Union, killed more than 800 people between 1968 and 2010, the year before it announced a permanent ceasefire.
The letter was sent to figures such as former United Nations secretary general Kofi Annan to reassure them that ETA was firm in its decision to disband, the representative said. "Some are still bleeding because suffering is not a thing of the past", the group said.
At a news conference Wednesday in the southern Spanish city of San Sebastian, Covite President Consuelo Ordonez criticized a statement last week in which ETA sought forgiveness from victims "who didn't have a direct role in the conflict".
Cowboys QB Dak Prescott says Jason Witten will be missed
The announcement of Witten's retirement comes later than the Cowboys would like, but luckily the NFL Draft hasn't concluded. But rest assured that I'm going to do everything in my power to help us be better as we move forward.
Some 300 ETA members are imprisoned in Spain, France, and Portugal and as many as 100 are still on the run, according to prisoners' families group Forum Social.
At least 358 crimes believed to involve ETA are unresolved, according to Covite, an association of victims, survivors and their relatives that is campaigning for ETA members to be held to account.
However, a report commissioned previous year by the region's government led by the Basque Nationalist Party (PNV) documented over 4000 alleged cases of torture by the security forces in relation to Eta between 1960 and 2014. Only 20 torture cases have seen court sentences. In response, the Spanish government ordered its dissolution.
In April 2017 Eta staged a disarmament ceremony in the south of France, and in a letter to various Basque organisations last month it said it wanted to building a future from a new starting point.
"It doesn't matter if they apologise or not to us, because we will never forgive Eta", said Alfonso Alonso, leader of the PP in the Basque region. The special powers were granted by Spain to the region in a new constitution in 1978.
With its support waning and stepped-up police operations on both sides of the Pyrenees undermining its ability to wage an armed struggle, ETA had already declared in 2011 a "definitive end" to its armed campaign.