40. As the Court has stated on a number of occasions (see, among other authorities, Isgrò v. Italy, judgment of 19 February 1991, Series A no. 194-A, p. 12, § 34, and Lüdi, cited above, p. 21, § 47), it may prove necessary in certain circumstances to refer to depositions made during the investigative stage (in particular, where a witness refuses to repeat his deposition in public owing to fears for his safety, a not infrequent occurrence in trials concerning Mafia-type organisations). If the defendant has been given an adequate and proper opportunity to challenge the depositions, either when made or at a later stage, their admission in evidence will not in itself contravene Article 6 §§ 1 and 3 (d). The corollary of that, however, is that where a conviction is based solely or to a decisive degree on depositions that have been made by a person whom the accused has had no opportunity to examine or to have examined, whether during the investigation or at the trial, the rights of the defence are restricted to an extent that is incompatible with the guarantees provided by Article 6 (see Unterpertinger v. Austria, judgment of 24 November 1986, Series A no. 110, pp. 14-15, §§ 31-33; Saïdi v. France, judgment of 20 September 1993, Series A no. 261-C, pp. 56-57, §§ 43-44; and Van Mechelen and Others, cited above, p. 712, § 55; see also Dorigo v. Italy, application no. 33286/96, Commission’s report of 9 September 1998, § 43, unpublished, and, on the same case, Committee of Ministers Resolution DH (99) 258 of 15 April 1999).