If you have probable cause, which I am sure they did, Clark just did not want to let out any details that may have heard the case. My dad is an ex APD detective and remembers interviewing him, along with 2398 other people. He was pretty happy to see this come to a close.
From what I gathered it was a combination to all the police work pointing to one guy, and the testimony of a relative that lead them to him.
edit for lafengas: Yes, yes he is guilty. DNA doesn’t lie, not in 3 separate cases. Now, does he get convicted? That is all dependent on the DAs case, and the police work dating back 20+ years…