“101. The Court's primary concern under Article 6 § 1 is to evaluate the overall fairness of the criminal proceedings (…). In making this assessment the Court will look at the proceedings as a whole, including the way in which the evidence was obtained, having regard to the rights of the defence but also to the interest of the public and the victims in seeing crime properly prosecuted (…) and, where necessary, to the rights of witnesses (…).
(...)
105. (...) the use as evidence of statements obtained at the stage of a police inquiry and judicial investigation is not in itself inconsistent with Article 6 §§ 1 and 3 (d), provided that the rights of the defence have been respected. As a rule, these rights require that the defendant be given an adequate and proper opportunity to challenge and question a witness against him — either when that witness is making his statements or at a later stage of the proceedings (…).
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107. According to the principles developed in the Al-Khawaja and Tahery judgment, it is necessary to examine in three steps the compatibility with Article 6 §§ 1 and 3 (d) of the Convention of proceedings in which statements made by a witness who had not been present and questioned at the trial were used as evidence (ibid., § 152). The Court must examine
(i) whether there was a good reason for the non-attendance of the witness and, consequently, for the admission of the absent witness's untested statements as evidence (ibid., §§ 119-25);
(ii) whether the evidence of the absent witness was the sole or decisive basis for the defendant's conviction (ibid., §§ 119 and 126-47); and
(iii) whether there were sufficient counterbalancing factors, including strong procedural safeguards, to compensate for the handicaps caused to the defence as a result of the admission of the untested evidence and to ensure that the trial, judged as a whole, was fair (ibid., § 147).
(...)
118. (...) it will, as a rule, be pertinent to examine the three steps of the Al-Khawaja-test in the order defined in that judgment (see paragraph 107 above). However, all three steps of the test are interrelated and, taken together, serve to establish whether the criminal proceedings at issue have, as a whole, been fair. (...)
(c) Principles relating to each of the three steps of the Al-Khawaja and Tahery test
(i) Whether there was a good reason for the non-attendance of a witness at the trial
119. Good reason for the absence of a witness must exist from the trial court’s perspective, that is, the court must have had good factual or legal grounds not to secure the witness’s attendance at the trial. If there was a good reason for the witness’s non-attendance in that sense, it follows that there was a good reason, or justification, for the trial court to admit the untested statements of the absent witness as evidence. There are a number of reasons why a witness may not attend a trial, such as (…) the witness’s unreachability.
120. In cases concerning a witness’s absence owing to unreachability, the Court requires the trial court to have made all reasonable efforts to secure the witness’s attendance (…). The fact that the domestic courts were unable to locate the witness concerned or the fact that a witness was absent from the country in which the proceedings were conducted was found not to be sufficient in itself to satisfy the requirements of Article 6 § 3 (d), which requires the Contracting States to take positive steps to enable the accused to examine or have examined witnesses against him (…). Such measures form part of the diligence which the Contracting States have to exercise in order to ensure that the rights guaranteed by Article 6 are enjoyed in an effective manner (…). Otherwise, the witness’s absence is imputable to the domestic authorities (…).
121. It is not for the Court to compile a list of specific measures which the domestic courts must have taken in order to have made all reasonable efforts to secure the attendance of a witness whom they finally considered to be unreachable (…). However, it is clear that they must have actively searched for the witness with the help of the domestic authorities, including the police (…) and must, as a rule, have resorted to international legal assistance where a witness resided abroad and such mechanisms were available (…).
122. The need for all reasonable efforts on the part of the authorities to secure the witness’s attendance at the trial further implies careful scrutiny by the domestic courts of the reasons given for the witness’s inability to attend trial, having regard to the specific situation of each witness (…).
(ii) Whether the evidence of the absent witness was the sole or decisive basis for the defendant’s conviction
123. As regards the question whether the evidence of the absent witness whose statements were admitted in evidence was the sole or decisive basis for the defendant’s conviction (second step of the Al-Khawaja and Tahery test), the Court reiterates that “sole” evidence is to be understood as the only evidence against the accused (…). “Decisive” evidence should be narrowly interpreted as indicating evidence of such significance or importance as is likely to be determinative of the outcome of the case. Where the untested evidence of a witness is supported by other corroborative evidence, the assessment of whether it is decisive will depend on the strength of the supporting evidence; the stronger the corroborative evidence, the less likely that the evidence of the absent witness will be treated as decisive (…).
124. As it is not for the Court to act as a court of fourth instance (…), its starting-point for deciding whether an applicant’s conviction was based solely or to a decisive extent on the depositions of an absent witness is the judgments of the domestic courts (…). The Court must review the domestic courts’ evaluation in the light of the meaning it has given to “sole” and “decisive” evidence and ascertain for itself whether the domestic courts’ evaluation of the weight of the evidence was unacceptable or arbitrary (…). It must further make its own assessment of the weight of the evidence given by an absent witness if the domestic courts did not indicate their position on that issue or if their position is not clear (….).”