The International Verification Commission (CIV) is due on Friday to relay a statement by ETA that "it can no longer use" the weapons in its power, sources briefed on the matter have told EL PAÍS.
This "significant gesture," as observers described it, could be followed by a direct announcement by ETA confirming its openness to an "unconditional" if "partial" laying down of weapons, after killing over 800 people and injuring thousands in its campaign of terror since 1968.
Although the Basque terrorist group said it was giving up armed violence in October 2011, so far it has not turned in its arsenal. Now, leaders appear to be committing to a gesture that would make it "impossible" to use the arms still in their power.
The formula partly mirrors steps taken by other terrorist groups, such as those in Northern Ireland and Colombia. But it is likely to be rejected by non-nationalists, who have never accepted any similarity between ETA and terrorism in other countries.
Counter-terrorism experts and politicians feel that this reference to "not being able to" use their weapons might mean ETA is willing to have its weapons caches sealed, and therefore to provide information about their locations to the verification committee.
But Basque nationalist circles are wary of just how much progress this new statement implies, considering ETA's delaying tactics ever since it announced its latest permanent ceasefire in October 2011.
The observers arrived in Bilbao on Thursday, although all contact with the press has been pushed back to Friday, when they are scheduled to hold a short press conference with no questions taken.