Senior ETA members including fugitives are expected to appear in a video declaring the end of their separatist campaign.
After more than four decades of violence, ETA is reportedly planning to mark its dissolution on Thursday with a filmed statement that could feature one of the Basque separatist group's most wanted fugitives.
Created in 1959 at the height of Francisco Franco's dictatorship, ETA waged a relentless campaign of killings and kidnappings in its fight for an independent Basque homeland in northern Spain and southwest France, leaving at least 829 dead.
Weakened in recent years by the arrest of its leaders, ETA announced a permanent ceasefire in 2011 and began formally surrendering its arms last year.
In a leaked letter published on Wednesday by Spanish online newspaper El Diario, it went a step further, announcing it was fully disbanding in what marks the end of western Europe's last armed insurgency.
"ETA has completely dissolved all of its structures and declared an end to its political initiative," it said in the letter addressed to various groups and figures involved in recent peace efforts.
On Thursday, ETA is expected to release a filmed declaration confirming the end of this dark chapter in Spanish history.
Spanish press have reported that Jose Antonio Urrutikoetxea, better known as Josu Ternera, will be in the video.
A veteran, high-level member of ETA, he is a wanted fugitive who disappeared in 2002. The 67-year-old is said to be suffering from cancer.
His potential presence in the video has infuriated some of ETA's victims, who say the separatist group should first and foremost condemn its history of violence and shed light on more than 350 unsolved crimes.
"This is not the end of ETA we deserved," said Consuelo Ordonez, head of the Covite victims' association, on Wednesday.
A partial apology by the separatist group last month in which it acknowledged the harm done and apologised to some of its victims – but not to those it considered legitimate targets such as police – has done little to stem the criticism.
While an overwhelming majority of Basques welcome the end of violence, many still want independence. The separatist coalition EH Bildu, the second-largest grouping in the regional parliament, won 21 percent of the votes in the 2016 regional elections.
Critics charge that Basque pro-independence parties, which include among their ranks people once part of or linked to ETA, are trying to impose their version of events.
Separatists argue that Basques have been repressed for decades, even centuries, by Spain and France.
That came to a head under Franco who forbade the use of the Basque language in public – cue ETA and its struggle against authority, which separatists see as a conflict.
Victims and historians, though, insist ETA was a terrorist group that went on killing even after Spain transitioned to democracy in the late 1970s. It is recognised as such by the European Union.
"ETA never wanted to bring about any democracy or any freedom, it wanted to end it," said Basque philosopher Fernando Savater.
"And that's what we want to remind people."
But apart from ETA victims, there were also separatists killed by far-right groups and death squads backed by members of Spain's security forces in what has become known as a "dirty war" campaign.
According to a December report commissioned by the Basque regional government, more than 4,100 complaints of police torture were made between 1960 and 2014.
Those victims are also demanding they be acknowledged.
"If you don't recognise part of the suffering, it's very difficult to create conditions for ... reconciliation," said Ane Muguruza, 28.
Her father Josu, a lawmaker for Herri Batasuna, ETA's political wing, was murdered in 1989 by far-right militants she believes were state-backed.
"It's very difficult when there are open wounds.
"To close them, you have to clean and cure them. And for that to happen, there must be acknowledgment, it's crucial."