My thoughts as well. But really, investigators screw up all the time, so would anybody really be that surprised if that had happened with this case as well?
It also should be remembered that at the time of the Ripper killings, detective work was in its infancy - and Forensic science was virtually unknown. This was 1888 - Scotland Yard didn't start using fingerprints, for example, until 1901.
Police basically relied on witnesses to catch anyone.