The case against Eta: It has targeted trains before

If Thursday's bombing was carried out by the Basque separatist organisation Eta, the militants would have had the motivation, the equipment and the sophistication.

Although Eta yesterday categorically denied responsibility for the commuter train bombs, the rucksacks in which some of the bombs were placed each contained 10.2 kilos of the Spanish-made explosive Titadine, Spain's Interior minister Angel Acebes said last night. Titadine has been used in previous Eta attacks, and a large quantity of the explosive was stolen from a French mine some years ago.

However, police also said yesterday that one of the unexploded bombs contained a copper detonator, whereas the detonators commonly used by Eta are made of aluminium.

The bombs, which had their alarms set for 7.39am, were detonated by mobile phone. "Some people say that this sort of thing would be beyond Eta's capabilities ­ but we need to be circumspect," said Jonathan Eyal, the director of the Royal United Services Institute. "It is a fallacy to believe that Eta were somehow amateurs." Mr Eyal recalled that in the final days of the Franco era, the then prime minister, Admiral Luis Carrero Blanco, was assassinated in Madrid by a bomb detonated as his armoured car passed by. "That was a very skilful operation ­ even 25 years ago Eta had the sophistication," he said.

The hallmarks of Eta bombings include a telephoned warning ahead of an attack. In the case of Thursday's bombing, no such warning was given. Eta claims invariably follow an attack, although not necessarily on the same day. Eta attacks also tend to be single, rather than the multiple, simultaneous "spectaculars" favoured by al-Qa'ida.

But Paul Heywood, an expert from Nottingham University, said the Basque separatist terrorists may have been trying to prove a point by carrying out such a large-scale atrocity. Spanish authorities had foiled what they said was an Eta attack planned for central Madrid, when they stopped a van containing half a ton of explosives, and detained two people on 29 February. It was also known that Eta operatives had been looking at railway stations as a possible target: at Christmas, Spanish police foiled an Eta attack on a train using rucksack bombs.

Throughout its 36-year struggle for independence, Eta has variously targeted the military or civilians to coincide with broader political aims. The timing of Thursday's attacks ­ only three days before Sunday's general election ­ appears to have an obvious political motive. The Eta thinking may be that its support would grow in its Basque homeland if a re-elected conservative government cracks down even further on separatists. The Aznar government has consistently refused Basque calls for a referendum on further regional powers. The Prime Minister has ruled out any negotiations and has banned Eta's political wing, Batasuna.

Could there be a link between al-Qa'ida operatives and Eta? Although there are known links among European organisations, Mr Eyal thought an alliance to be extremely unlikely. "There would be a monumental backlash against Eta in the Basque country if it allied itself with al-Qa'ida," he said.