Your edit removes a vital piece of info, that the person whose DNA was found at the scene was not one of the five. The ops statement makes it clear the DNA comes from a sixth person, while your edit doesn't. In fact I don't even see how you asked your question.
> The only person who's DNA was found there (who wasn't part of the five) is the person who claims that there was only one attacker (he confessed to the crime and made the claim that he was the only attacker after meeting one of the five in prison).
Should be:
> The only person whose DNA was found there is the person who claims that there was only one attacker (he confessed to the crime and made the claim that he was the only attacker after meeting one of the five in prison).
I would argue that your comments in brackets are also irrelevant, because it appears to be suggesting something for which there is no basis for. In other words, he confessed to the crime, period. And his DNA was found on the victim. So we know he is guilty, period.