Groups claiming responsibility for the attack included
Basque separatist group
ETA, the
First of October Anti-Fascist Resistance Groups (GRAPO), Unity of the Abu Zeinab Martyrs, Wa'd (a front of the
PFLP-SC) and the
Islamic Jihad Organization. The callers from ETA claimed the bomb had gone off earlier than planned and that the bombing was meant to target American soldiers who would have been in the restaurant later, and apologised to the victims of the bombing. After first blaming ETA,
Spanish Interior Minister José Barrionuevo concluded the Islamic Jihad Organization and Wa'd had the most credible claim of responsibility following investigations by the National Police. The claim by Wa'd, distributed by the
Kuwait News Agency was noted as it included the anagram of the sugar envelopes from the El Descanso restaurant. ETA was also known to have had ties to extremists in
Lebanon and
Syria where they had access to training camps, and investigators suspected links between Islamic Jihad, ETA and GRAPO. Islamic Jihad had itself been implicated in several attacks in Spain the previous year, including a
machine gun attack on a
Kuwaiti newspaper publisher and the killing and wounding of two Saudi Arabians, both attacks in
Marbella, and the shooting of a Lebanese embassy employee in Madrid. According to the
El País newspaper and the Group of Strategic Studies, which cite the attack as "the first attack of Islamist terrorism in Spain," the credibility of Islamic Jihad eventually gained weight, also as a letter circulated two weeks after the bombing stated: "Islam is ready. Spain and Italy are the first targets. The attack in Madrid has been the beginning of the Islamic holy war. Death to the United States. The apostles of death are ready to resume the holy war." ==Later developments==