Can a former witness be used as evidence in a criminal case?

Evidence Code section 1291 permits the use of former testimony when a witness is unavailable.¹⁸ Section 240, subdivision (a)(5) states that a witness is unavailable if "[a]bsent from the hearing and the proponent of his or her statement has exercised reasonable diligence but has been unable to procure his or her attendance by the court's process." "The term 'reasonable diligence'... ' "connotes persevering application, untiring efforts in good earnest, efforts of a substantial character. [Citations.]" ' [Citation.] Considerations relevant to this inquiry include the timeliness of the search, the importance of the proffered testimony, and whether leads of the witness's possible location were competently explored. [Citation.] We independently review a trial court's due diligence determination." (People v. Wilson (2005) 36 Cal.4th 309, 341.)