International Criminal Police Organization had come into existence in the 1920s, as a records clearing house, so that the police of one country could communicate information with another and had steadily grown in power.
Since the nineties, it rode the wave of the World Wide Web and became the international enforcer of Internet law. When almost everything was run through online networks in almost every modernized country, Interpol statutes were considered the final word.
Interpol also had priority with old-fashioned criminals who crossed borders. Only when it suited them, they'd allow local authorities to handle matters.
Interpol was looking into Roy O'Brien's murder in the NSPD and went through the police report.