Could this be the case with Thursday’s bombings in Madrid? Has Eta developed links with al-Qaeda, or a similar group, to create an alternative infrastructure to the one which had apparently been virtually dismantled by police successes over the last four years?

Might such links resolve the conundrum that the attacks bear some of the hallmarks of both groups, yet do not quite fit the modus operandi of either?

This scenario is necessarily speculative, and, on the face of it, still the most unlikely given the facts to have emerged so far. But an open mind is perhaps the greatest asset in assessing the entirely new situation presented by what we may now call El 11 de M arzo, or March 11.

The hypothesis that this was a joint Eta-Islamist operation at least enables us to tease out the distinctive features of both groups, and assess a little more clearly who was most likely to be responsible for the atrocities.

Unfortunately open minds have not been on display in the highly politicised debate about the likely culprits. As late as yesterday morning, Ana Palacio, the Foreign Minister, was insisting that it “crystal clear” that Eta was responsible. Almost simultaneously Koldo Gorostiaga, an MEP of the Batasuna party which is Eta’s political wing, was telling me it was “completely impossible” that Eta could have carried out the attack.

Both statements are highly political. Spain votes tomorrow in a general election. If the attacks are blamed on Eta, the Spanish people are likely to swing to the ruling party, which has always presented a tough and united front against the Basque terrorists. If the bombings were carried out by an Islamist group, then the gulf between the Spanish public and the Aznar Government over its support for the Iraq war is likely to reopen.

For supporters of Basque independence such as Señor Gorostiaga, Eta involvement in the attacks would be a political disaster. Eta’s return to violence after the 1998-99 ceasefire cost his Batasuna party

50 per cent of its electorate. Those who do still give tacit support to Eta tend to do so reluctantly, out of old loyalties, and would certainly rather see a more “selective armed struggle” than Thursday’s descent into indiscriminate slaughter.

But to an independent observer the only thing that is clear about the massacre is that nothing is yet certain, though the balance of evidence is swinging towards the involvement of Islamic terrorists.

The indictment sheet against Eta includes: likelihood (most bombings in Madrid are down to it); immediate precedent (it tried to bomb Madrid only ten days ago); historical precedent (it usually carries out “spectacular” attacks at election time); motive (a desperate attempt to show that the Spanish police had not pulled its teeth); some forensic scientific evidence (the Spanish police claim that the explosives used are similar to Eta’s standard issue).

The charge sheet against an Islamist group runs like this: modus operandi (simultaneous and highly co-ordinated attacks); motive (Spain’s support for Bush’s Iraq war); scale (a large number of civilian casualties fits an apocalyptic campaign which, unlike Eta’s, is totally impervious to Western public opinion); forensic scientific evidence (the Koranic tapes found in a truck associated with the bombings); admission (the e -mail to al-Quds claiming to come from the al-Qaeda-associated Abu Hafs al-Masri Brigades, and Eta’s unprecedented denial of responsibility last night).

What, then, is the likelihood that the two groups were working together to produce this confusing jigsaw of evidence?

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