What you've described amounts to reasonable suspicion, but falls short of probable cause. So, no prosecution would be likely at this point. In fact, you've described multiple crimes perpetrated/supported by a multi-party conspiracy--which has always been very difficult to prove (cattle rustling, like horse stealing, was a capital offense).
Even in the 1870s, your investigator would need some sort of direct evidence to seek an indictment & subsequent prosecution--like a co-conspirator confessing, providing corroborative (hard) evidence, and testifying in court.
Alternatively, an incensed mob with a rope tended not to be so hamstrung by such civilized procedures.