he didn't. The Ruelas DNA being "found", along with Leiterman's, in the Mixer sample are most certainly a consequence of contamination of some sort.
Aside from the obvious problem with Ruelas being only four years old at the time Jane Mixer was killed, the thing that really points towards a contaminated sample is this---
In the sample in question, from the Mixer evidence, both the DNA of Leiterman and Ruelas come out as strong, non-degraded...Mixer's DNA is obviously degraded "weak" in samples tested, which would be consistant with a thirty-three year old sample stored in the non-refigerated storage (common back in the sixties). Why would her dna profile come out weak, yet Ruelas's and Lieterman's DNA have no apparent degredation? The obvious reason is a newly-contaminated sample.
Lets not forget that both Ruelas and Leiterman had their DNA processed at the very same lab as the Mixer sample was, either on or about the same day (Ruelas sample was processed the same day in the lab---the sample from Leiterman is not as clear from sources I checked, but it was within the same week anyhow, if not within a day or two, like the Ruelas sample).
So, those who say that Ruelas HAD to had been involved in the murder of Jane Mixer back in 1969---whats more incredible---a coincidence of absolutely astronomical proportions involving Mixer's sample being processed--on the same day--as Ruelas's sample,. some 33 years later??!! ...and Leiterman's, also the same week....just so happens each was in the lab that week? Please, the much more likely explaination is contamination. I think available samples need to be retested. Why was this not done? Somewhere along the chain, after the Mixer evidence was taken from storage and while being processed in the lab, it was contaminated.
Lab contamination happens more often than many may realize---it's seldom a cronic problem, but it does happen every now and then and the press certainly is not alerted to this each time it does happen. The State must do what is right and retest the evidence, whatever may be left that can be considered reliable. Cases this old are notoriously difficult to solve.