In a letter dated 7th April, Euskadi Ta Askatasuna — or ETA — whose name stands for "Basque Homeland and Freedom," declared that "after giving up all its weaponry (arms and explosives) to Basque civil society representatives [ETA] now is a disarmed organization."
If true, it would amount to a significant turning point in a civil struggle that has stretched back more than 40 years.
The group first emerged in the 1960s as a resistance movement opposing General Franco's repressive military dictatorship, which attempted to repress local Basque culture and traditions. The Basque language was banned and dissenters imprisoned and tortured.
After the death of General Franco in 1975, Spain's Basque country was granted more autonomy than any other Spanish region, with its own police force and parliament in control of taxes and education.
However, ETA continued to push for an independent Basque state for the seven regions in northern Spain and southern France that Basque separatists claim as their own.
More than 820 people have been killed in a bloody terror campaign, including police and politicians, with thousands wounded.
The high body count and bitterness that has been entrenched in the political dispute for so many years has made diplomatic progress difficult to achieve.
The Spanish government refuses to negotiate with ETA, and both France and Spain have arresting hundreds of militants in recent years, including key leadership figures.
The news that ETA intends to disarm has been met with a mix of wariness and outright suspicion in some parts of Spain.
ETA has declared at least two ceasefires before, but abandoned both.
And some victims are calling on the Spanish government to ensure that ETA atrocities do not go unpunished.
One local politician, and member of the Basque Parliament, Gorka Maneiro Labayen, tweeted:
ETA vuelve q demostrar q es basura. No cogisteis las armas para defender al pueblo sino para matarnos y acabar con nuestra libertad pic.twitter.com/ZaD8K6oCSk— Gorka Maneiro (@Gorka_Maneiro) April 7, 2017
"ETA returns to demonstrate that it is garbage. You did not take up arms to defend the people but to kill us and end our freedom."
An online petition, demanding that ETA terrorists pay for their crimes has more than 12,000 signatories.
Meanwhile, ETA's letter warns that their disarmament plan "is not [yet] completed."
"We want to warn that still the process can be attacked by the enemies of peace. The only real guarantee to succeed are the thousands of people gathering tomorrow in Bayonne [south-western France] supporting the disarmament."