Règle correspondante
Practice Relating to Rule 161. International Cooperation in Criminal Proceedings
Section A. Cooperation between States
Spain’s LOAC Manual (1996) provides: “States shall provide each other with the greatest possible mutual assistance for the penal repression of violations, at national and international level.” 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Publicación OR7-004, 2 Tomos, aprobado por el Estado Mayor del Ejército, Division de Operaciones, 18 March 1996, Vol. I, § 11.8.b.(4).
Spain’s LOAC Manual (2007) states:
States also undertake to act jointly or individually, in co-operation with the United Nations and in conformity with the Charter of the United Nations, to deal with serious violations of the [1949 Geneva] Conventions and the [1977 Additional] Protocols. 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Tomo 1, Publicación OR7–004, (Edición Segunda), Mando de Adiestramiento y Doctrina, Dirección de Doctrina, Orgánica y Materiales, 2 November 2007, § 11.3.b.(1).
In 2010, in the Couso case, which concerned the killing of a Spanish journalist in Baghdad on 8 April 2003 by troops of the United States of America, the Criminal Chamber of Spain’s Supreme Court held:
E) Even admitting for purely dialectic purposes that doubts could exist … concerning the rational indications of an offence found by the examining magistrate, it would still be probable that an offence was committed, which would thus have to be determined in an oral trial.
F) … [T]he Treaty on Mutual Judicial Assistance on criminal matters between Spain and the US, signed on 20 November 1990 and ratified on 23 April 1993, relating to Article 27 of the [1969] Vienna Convention on the Law of Treaties [on the observance of treaties], [has been manifestly unfulfilled by the US]. 
Spain, Supreme Court, Couso case, Judgment, 13 July 2010, Section II(II), Segundo, § 1, p. 7; see also Tercero, §§ 1–2, p. 9 and Quinto, §§ 1–2, pp. 10–11.
[emphasis in original]
The Court upheld the appeal against the order of 23 October 2009 by the Third Section of the Criminal Chamber of the Spanish National Court, which declared the termination of the proceedings, and held that “the proceedings must continue, and the outstanding preparatory enquiries must be undertaken, as well as any others arising from the clarification of the events under investigation.” 
Spain, Supreme Court, Couso case, Judgment, 13 July 2010, Section III, pp. 20–21.