The armed group is blamed for the deaths of 829 people in a four-decade campaign of bombings and shootings in pursuit of an independent homeland in northern Spain and southwestern France.
It declared a ceasefire on October 20, 2011.
"Eta is not dead, it still has weapons, explosives... but I don't think they will take up arms again," said Javier Zaragoza, chief prosecutor at the National Court, which handles terrorism cases.
"There are still 50 people left who are being investigated for belonging to a terrorist group or collaborating, or for participating in other offences" linked to Eta, he told AFP.
Zaragoza added that up to twenty members are still believed to be at large, living in hiding.
In contrast, some 90 people are in prison awaiting trial for acts linked to jihadism and around 30 have been sentenced and jailed.
Ten of these are serving time for their role in train bombings in Madrid in March 2004 that left 191 dead in what is still the worst extremist attack in Spain's history.
Close to a quarter of all investigations launched by the National Court are now linked to jihadists, said Zaragoza.
Spain in jihadist sights
Spain has so far been spared another attack and holiday-makers fleeing other restive destinations have flocked to the country known for its sunshine and beaches.
"There have been risky situations. We identified two or three cases of cells that were ready to act," said Zaragoza.
"But the attacks on March 11, 2004 taught us a lot," he said, adding that the focus was on prevention, with the adoption in 2015 of a law allowing authorities to detain people who had merely consulted jihadist websites.
But Zaragoza warned that Spain had recently started to be mentioned on jihadist websites.
"Spain is one of the countries they have in their sights for historical reasons as it was once Al-Andalus," he said, referring to a territory that was under Muslim rule from 711 to 1492.
But unlike France or Belgium, the country is less exposed to a "very high risk" phenomenon - the return of nationals who went to fight abroad and plan to commit extremist acts on home soil, he said.
Only around 200 Spaniards are estimated to have gone abroad to fight, compared to thousands from nearby France and Belgium.
And while Spain remains on high terror alert - level four out of five - Zaragoza warned against descending into alarmism.
"We must be careful," he said.
"One of the keys to the expansion of terrorism is to be present all the time in the media.
"If you constantly give news about their actions, you are in a way giving them what they want: generating terror among the people.
"Without the media, terrorism is nothing."
No negotiations with Eta
Eta meanwhile has sought to negotiate its dissolution with Spain and France in exchange for an amnesty or for gathering together in Basque prisons the roughly 350 jailed Eta members scattered across both countries.
Relatives regularly stage protests demanding they be held closer to home, but so far the government has taken a hard line.
"These negotiations cannot take place," said Zaragoza.
"You can't put a terrorist organisation on the same level as a state governed by the rule of law."
By Michaela Cancela-Kieffer