Why in harry potter and the prisoner of askaban is sirius arrested without using prior incarem?

In the the third book in the Harry Potter series, Sirius Black was arrested for Peter Pettigrew's death and other muggles deaths without using the spell prior incarem to confirm it... despite the fact that in the Fourth book, the only way that one could discover that Winky the house elf in the Fourth book was believed to have used Harry Potter's wand to conjure the dark mark was thru the spell prior incarem.