Not a lot of empirical work on the practices of inter/transnational information and intelligence sharing has been undertaken. A notable exception is James Sheptycki's study of police cooperation in the English Channel region (2002), which provides a systematic content analysis of information exchange files and a description of how these transnational information and intelligence exchanges are transformed into police case-work. The study showed that transnational police information sharing was routinized in the cross-Channel region from 1968 on the basis of agreements directly between the police agencies and without any formal agreement between the countries concerned. By 1992, with the signing of the Schengen Treaty, which formalized aspects of police information exchange across the territory of the European Union, there were worries that much, if not all, of this intelligence sharing was opaque, raising questions about the efficacy of the accountability mechanisms governing police information sharing in Europe (Joubert and Bevers, 1996).

Answer this question, if possible (if impossible, reply "unanswerable"): When did Sheptycki write about police cooperation?
2002