Didn't Fudge come into office around the time Harry started Hogwarts (OoP 10: "elected 5 years ago")? So wasn't it indeed Crouch who incarcerated Sirius sans trial? Crouch, Jr. wasn't revealed as a death eater until he joined Bella and the Lestrange brothers in torturing Nevilles' parents -- subsequent to the destruction of Voldemort and Sirius's arrest (GF 30; cf. HP Lexicon timeline)